ath9k: Fix kernel panic caused by invalid rate index access.

With the recent tx status optimization in mac80211, we bail out as
and and when invalid rate index is found. So the behavior of resetting
rate idx to -1 and count to 0 has changed for the rate indexes that
were not part of the driver's retry series.

This has resulted in ath9k using incorrect rate table index which
caused the system to panic. Ideally ath9k need to loop only for the
indexes that were part of the retry series and so simply use hw->max_rates
as the loop counter.

Pasted the stack trace of the panic issue for reference.

[  754.093192] BUG: unable to handle kernel paging request at ffff88046a9025b0
[  754.093256] IP: [<ffffffffa02eac49>] ath_tx_status+0x209/0x2f0 [ath9k]
[  754.094888] Call Trace:
[  754.094903]  <IRQ>
[  754.094928]  [<ffffffffa051f883>] ieee80211_tx_status+0x203/0x9e0 [mac80211]
[  754.094975]  [<ffffffffa053e305>] ? __ieee80211_wake_queue+0x125/0x140 [mac80211]
[  754.095017]  [<ffffffffa02e66c9>] ath_tx_complete_buf+0x1b9/0x370 [ath9k]
[  754.095054]  [<ffffffffa02e6fcf>] ath_tx_complete_aggr+0x51f/0xb50 [ath9k]
[  754.095098]  [<ffffffffa05382a3>] ? ieee80211_prepare_and_rx_handle+0x173/0xab0 [mac80211]
[  754.095148]  [<ffffffff81350e62>] ? _raw_spin_unlock_irqrestore+0x32/0x40
[  754.095186]  [<ffffffffa02e9735>] ath_tx_tasklet+0x365/0x4b0 [ath9k]
[  754.095224]  [<ffffffff8107a2a2>] ? clockevents_program_event+0x62/0xa0
[  754.095261]  [<ffffffffa02e2628>] ath9k_tasklet+0x168/0x1c0 [ath9k]
[  754.095298]  [<ffffffff8105599b>] tasklet_action+0x6b/0xe0
[  754.095331]  [<ffffffff81056278>] __do_softirq+0x98/0x120
[  754.095361]  [<ffffffff8100cd5c>] call_softirq+0x1c/0x30
[  754.095393]  [<ffffffff8100efb5>] do_softirq+0x65/0xa0
[  754.095423]  [<ffffffff810563fd>] irq_exit+0x8d/0x90
[  754.095453]  [<ffffffff8100ebc1>] do_IRQ+0x61/0xe0
[  754.095482]  [<ffffffff81351413>] ret_from_intr+0x0/0x15
[  754.095513]  <EOI>
[  754.095531]  [<ffffffff81014375>] ? native_sched_clock+0x15/0x70
[  754.096475]  [<ffffffffa02bcfa6>] ? acpi_idle_enter_bm+0x24d/0x285 [processor]
[  754.096475]  [<ffffffffa02bcf9f>] ? acpi_idle_enter_bm+0x246/0x285 [processor]
[  754.096475]  [<ffffffff8127fab2>] cpuidle_idle_call+0x82/0x100
[  754.096475]  [<ffffffff8100a236>] cpu_idle+0xa6/0xf0
[  754.096475]  [<ffffffff81339bc1>] rest_init+0x91/0xa0
[  754.096475]  [<ffffffff814efccd>] start_kernel+0x3fd/0x408
[  754.096475]  [<ffffffff814ef347>] x86_64_start_reservations+0x132/0x136
[  754.096475]  [<ffffffff814ef451>] x86_64_start_kernel+0x106/0x115
[  754.096475] RIP  [<ffffffffa02eac49>] ath_tx_status+0x209/0x2f0 [ath9k]

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>