fwohci: Cast bitfield to uint32_t before passing it to roundup2().

The fallback for align_up() used by roundup2() uses typeof__()
which doesn't work for bitfields. This fixes the build on GCC which
uses the fallback.

Reviewed by: arichardson, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D28599