bitfield: avoid gcc-8 -Wint-in-bool-context warning

Passing an enum into FIELD_GET() produces a long but harmless warning on
newer compilers:

                   from include/linux/linkage.h:7,
                   from include/linux/kernel.h:7,
                   from include/linux/skbuff.h:17,
                   from include/linux/if_ether.h:23,
                   from include/linux/etherdevice.h:25,
                   from drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c:63:
  drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c: In function 'iwl_mvm_rx_mpdu_mq':
  include/linux/bitfield.h:56:20: error: enum constant in boolean context [-Werror=int-in-bool-context]
     BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero"); \
                      ^
  ...
  include/linux/bitfield.h:103:3: note: in expansion of macro '__BF_FIELD_CHECK'
     __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
     ^~~~~~~~~~~~~~~~
  drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c:1025:21: note: in expansion of macro 'FIELD_GET'
      le16_e...