net: macb: Add __nonstring annotations for unterminated strings

When a character array without a terminating NUL character has a static
initializer, GCC 15's -Wunterminated-string-initialization will only
warn if the array lacks the "nonstring" attribute[1]. Mark the arrays
with __nonstring to correctly identify the char array as "not a C string"
and thereby eliminate the warning:

In file included from ../drivers/net/ethernet/cadence/macb_main.c:42:
../drivers/net/ethernet/cadence/macb.h:1070:35: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
1070 | GEM_STAT_TITLE(TX1519CNT, "tx_greater_than_1518_byte_frames"),
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/cadence/macb.h:1050:24: note: in definition of macro 'GEM_STAT_TITLE_BITS'
1050 | .stat_string = title, \
| ^~~~~
../drivers/net/ethernet/cadence/macb.h:1070:9: note: in expansion of macro 'GEM_STAT_TITLE'
1070 | GEM_STAT_TITLE(TX1519CNT, "tx_greater_than_1518_byte_frames"),
| ^~~~~~~~~~~~~~
../drivers/net/ethernet/cadence/macb.h:1097:35: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
1097 | GEM_STAT_TITLE(RX1519CNT, "rx_greater_than_1518_byte_frames"),
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/cadence/macb.h:1050:24: note: in definition of macro 'GEM_STAT_TITLE_BITS'
1050 | .stat_string = title, \
| ^~~~~
../drivers/net/ethernet/cadence/macb.h:1097:9: note: in expansion of macro 'GEM_STAT_TITLE'
1097 | GEM_STAT_TITLE(RX1519CNT, "rx_greater_than_1518_byte_frames"),
| ^~~~~~~~~~~~~~

Since these strings are copied with memcpy() they do not need to be
NUL terminated, and can use __nonstring:

memcpy(p, gem_statistics[i].stat_string,
ETH_GSTRING_LEN);

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117178 [1]
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250312200700.make.521-kees@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>