ath9k: Proper TX99 interrupt ref count

On TX99 mode, instead of assuming interrupt mask non ATH9K_INT_GLOBAL,
let ath9k_hw_disable_interrupts proper set interrupt ref count.
This prevents some PCI PERR occurring specialy when setting 11b and n rates.

Signed-off-by: Eduardo Abinader <eduardo.abinader@riverbed.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>