HE: Fix set_he_cap() parsing of config options for MU EDCA Params

HE: Fix set_he_cap() parsing of config options for MU EDCA Params

When I replaced the POS() function with ffs() when applying relevant
parts from the original patch, this ended up breaking the frame
construction since the POS() function was supposed to count the bit
offset for the mask with 0 being the LSB instead of 1 returned by ffs().
Furthermore, ffs() is not available in all C libraries (e.g., not
directly exposed by strings.h on Android), so better not depend on that
or compiler builtins for this since there is no need for this to be as
fast as possible in configuration parsing.

Fix this with a simple function to determine the number of bits the
value needs to be shifted left to align with the mask.

Fixes: 11ce7a1bc3e2 ("HE: Add MU EDCA Parameter Set element (AP)")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>