ath9k: do not enable interrupt on set interrupt mask

At preset set_interrupt also enables interrupt after changing
mask. This is not necessary in all cases and also sometime it
breaks the assumption that interrupt was disabled. So let us
enable the interrupt explicity if it was disabled earlier.
This could also avoid unnecessary register ops and also helps
the follow up patch to have global ref count for interrupts ops.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>