mwl8k: fix changed flags handling in mwl8k_bss_info_changed()

Previously, mwl8k_bss_info_changed() would refuse to do anything if
the 'changed' argument indicated that the association status hadn't
changed.  Fix this up so that it will allow changing things like the
preamble type, the slot time and the CTS-to-self protection method
without having to reassociate.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>