ath9k: Fix IRQ nobody cared issue with ath9k

IRQs should be disabled before calling free_irq. Also flush pending
IRQs. Pasted the kernel log message for reference.

kernel: irq 17: nobody cared (try booting with the "irqpoll" option)
kernel:  [<c0252d2c>] __report_bad_irq+0x2e/0x6f
kernel:  [<c0252f22>] note_interrupt+0x1b5/0x207
kernel:  [<c025258b>] ? handle_IRQ_event+0x21/0x48
kernel:  [<c02534cb>] handle_fasteoi_irq+0x8e/0xad
kernel:  [<c0205650>] do_IRQ+0x6c/0x84
kernel:  [<c020425f>] common_interrupt+0x23/0x28
kernel:  [<c034f6f6>] ? acpi_idle_enter_simple+0x198/0x205
kernel:  [<c044686c>] ? menu_select+0x5c/0x78
kernel:  [<c0445a95>] cpuidle_idle_call+0x59/0x89
kernel:  [<c02029d7>] cpu_idle+0xae/0xcf
kernel:  [<c0543102>] rest_init+0x4e/0x50
kernel:  =======================
kernel: handlers:
kernel: [<f88fdd26>] (ath_isr+0x0/0x13a [ath9k])
kernel: Disabling IRQ #17

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Tested-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>