eth: fbnic: fix s390 build.

Building the fbnic nn s390, yield a build bug:

In function ‘fbnic_config_drop_mode_rcq’,
    inlined from ‘fbnic_enable’ at drivers/net/ethernet/meta/fbnic/fbnic_txrx.c:1836:4:
././include/linux/compiler_types.h:510:45: error: call to ‘__compiletime_assert_919’ declared with attribute error: FIELD_PREP: value too large for the field

The relevant mask is 9 bits wide, and the related value is the cacheline
aligned size of struct skb_shared_info.

On s390 the cacheline size is 256 bytes, and skb_shared_info minimum
size on 64 bits system is 320 bytes.

Avoid building the driver for such arch.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/202407170432.dYJQOWVz-lkp@intel.com/
Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: 0cb4c0a1

 ("eth: fbnic: Implement Rx queue alloc/start/stop/free")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/5dfefd3e90e77828f38e68854b171a5b8b8c6ede.1721215379.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>