net: enetc: fix implicit declaration of function FIELD_PREP

The kernel test robot reported the following error:

drivers/net/ethernet/freescale/enetc/ntmp.c: In function 'ntmp_fill_request_hdr':
drivers/net/ethernet/freescale/enetc/ntmp.c:203:38: error: implicit
declaration of function 'FIELD_PREP' [-Wimplicit-function-declaration]
203 | cbd->req_hdr.access_method = FIELD_PREP(NTMP_ACCESS_METHOD,
| ^~~~~~~~~~

Therefore, add "bitfield.h" to ntmp_private.h to fix this issue.

Fixes: 4701073c3deb ("net: enetc: add initial netc-lib driver to support NTMP")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505101047.NTMcerZE-lkp@intel.com/
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>