ath5k: Update interrupt masking code

*Properly get/set all available ISR/IMR values and review common/uncommon bits
*Better handling of per-txq interrupts (we can now resolve what q is generating
 each interrupt -this will help in debuging wme later)
*Some minor updates from legacy-hal
*Properly handle RXNOFRM and TXNOFRM interrupt masking (even when we don't set
 them on IMR they keep showing up, so we disable them by zeroing AR5K_RXNOFRM
 and AR5K_TXNOFRM registers). This doesn't exist on legacy-hal but i've tested
 it on various cards and it works fine.

Changes-Licensed-under: ISC
Signed-Off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>