wifi: b43: silence sparse warnings

sparse complains on this code about casts that lose bits
due to the usage of bitwise not, but really we do want
16 bits only, so clarify that by using masks.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240223114023.c64e2d348453.Iccc4ace1116721a044e5f31f40ea7709e72145f3@changeid