staging: wlan-ng: use GENMASK macro in two bitwise operations in prism2sta.c

This patch replace actual mask stuff using BIT macros with
or operators to make use of GENMASK macro which simplifies
code clearity and readibility.

It applies for two bitwise operations included in prism2sta.c source file.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>