brcmsmac: remove brcms_bss_cfg->BSS

This was a read only member. The checks using BSS are replaced by
better fitting checks of the new type member.

The change in brcms_c_tbtt() was based on code from b43, in
brcms_c_ps_allowed() the same happens with BSS being true or false,
beaconing and probe responses are just needed in ap mode.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>