ath9k: use per-device struct for pm_qos_* operations

ath9k: use per-device struct for pm_qos_* operations

The ath9k driver uses a shared pm_qos_request_list structure for all
devices. This causes the following warning if more than one device is
present in the system:

WARNING: at kernel/pm_qos_params.c:234 ath9k_init_device+0x5e8/0x6b0()
pm_qos_add_request() called for already added request
Modules linked in:
Call Trace:
[<802b1cdc>] dump_stack+0x8/0x34
[<8007dd90>] warn_slowpath_common+0x78/0xa4
[<8007de44>] warn_slowpath_fmt+0x2c/0x38
[<801b0828>] ath9k_init_device+0x5e8/0x6b0
[<801bc508>] ath_pci_probe+0x2dc/0x39c
[<80176254>] pci_device_probe+0x64/0xa4
[<8019471c>] driver_probe_device+0xbc/0x188
[<80194854>] __driver_attach+0x6c/0xa4
[<80193e20>] bus_for_each_dev+0x60/0xb0
[<80193580>] bus_add_driver+0xcc/0x268
[<80194c08>] driver_register+0xe0/0x198
[<801764e0>] __pci_register_driver+0x50/0xe0
[<80365f48>] ath9k_init+0x3c/0x6c
[<8006050c>] do_one_initcall+0xfc/0x1d8
[<80355340>] kernel_init+0xd4/0x174
[<800639a4>] kernel_thread_helper+0x10/0x18

---[ end trace 5345fc6f870564a6 ]---

This patch fixes that warning by using a separate pm_qos_request_list
sructure for each device.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>