bwi: Fix clang 14 warning about possible unaligned access

[dim]

Summary

On architectures with strict alignment requirements (e.g. arm), clang 14
warns about a packed struct which encloses a non-packed union:

In file included from sys/dev/bwi/bwimac.c:79:
sys/dev/bwi/if_bwivar.h:308:7: error: field iv_val within 'struct bwi_fw_iv' is less aligned than 'union (unnamed union at sys/dev/bwi/if_bwivar.h:305:2)' and is usually due to 'struct bwi_fw_iv' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]
        }                       iv_val;
                                ^

It appears to help if you also add __packed to the inner union (i.e.
iv_val). No change to the layout is intended.