Ath5k: mask out unneeded interrupts

Mask out previously demanded interrupt flags because we set
new ones. Don't allow mixing them after switch from sta to
ibss and vice versa.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>