carl9170: fix sleep in softirq context

This patch fixes the following bug:

usb 1-1.1: restart device (8)
BUG: sleeping function called from invalid context at drivers/usb/core/urb.c:654
in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper
 (usb_poison_urb+0x1c/0xf8)
 (usb_poison_anchored_urbs+0x48/0x78)
 (carl9170_usb_handle_tx_err+0x128/0x150)
 (carl9170_usb_reset+0xc/0x20)
 (carl9170_handle_command_response+0x298/0xea8)
 (carl9170_usb_tasklet+0x68/0x184)
 (tasklet_hi_action+0x84/0xdc)

this only happens if the device is plugged in an USB port,
the driver is loaded but inactive (e.g. the wlan interface
is down). If the device is active everything is fine.

Signed-off-by: Ronald Wahl <ronald.wahl@raritan.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>