net: stmmac: xgmac: use const char arrays for string constants

Jiri Slaby advises me that the preferred mechanism for declaring
string constants is static char arrays, so use that here.

This mostly reverts
commit 1692b9775e74 ("net: stmmac: xgmac: use #define for string constants")

That commit was a fix for
commit 46eba193d04f ("net: stmmac: xgmac: fix handling of DPP safety error for DMA channels").
The fix being replacing const char * with #defines in order to address
compilation failures observed on GCC 6 through 10.

Compile tested only.
No functional change intended.

Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/netdev/485dbc5a-a04b-40c2-9481-955eaa5ce2e2@kernel.org/
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>