LinuxKPI: add BITS_TO_BYTES()

Just like BITS_TO_LONG() ... used in rtw89.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D44603

(cherry picked from commit f674f016c03d49e14f1c651ed088a6f2d01a62ee)