bna: bfi.h: Avoid the use of one-element array

bna: bfi.h: Avoid the use of one-element array

One-element arrays are being deprecated[1]. Replace the one-element
array with a simple value type 'u8 rsvd'[2], once it seems this is
just a placeholder for alignment.

[1] https://github.com/KSPP/linux/issues/79
[2] https://github.com/KSPP/linux/issues/86

Tested-by: kernel test robot <lkp@intel.com>
Link: https://github.com/GustavoARSilva/linux-hardening/blob/master/cii/0-day/bfi-20200718.md
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>