staging: rtl8723au: core: Replace open coded version with existing hweight function

This patch replaces the open coded version with Linux's existing hweight
functions.
The original version checked each of the low four bits, accumulating and
returning the result. Instead, use a mask to select those four bits, and
pass the result to hweight8.
It also makes the code in this function simpler.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>