rtl8187: ->brightness_set can not sleep

Fix:

BUG: sleeping function called from invalid context at kernel/workqueue.c:2547
in_atomic(): 1, irqs_disabled(): 0, pid: 629, name: wpa_supplicant
2 locks held by wpa_supplicant/629:
 #0:  (rtnl_mutex){+.+.+.}, at: [<c08b2b84>] rtnl_lock+0x14/0x20
 #1:  (&trigger->leddev_list_lock){.+.?..}, at: [<c0867f41>] led_trigger_event+0x21/0x80
Pid: 629, comm: wpa_supplicant Not tainted 3.3.0-0.rc3.git5.1.fc17.i686
Call Trace:
 [<c046a9f6>] __might_sleep+0x126/0x1d0
 [<c0457d6c>] wait_on_work+0x2c/0x1d0
 [<c045a09a>] __cancel_work_timer+0x6a/0x120
 [<c045a160>] cancel_delayed_work_sync+0x10/0x20
 [<f7dd3c22>] rtl8187_led_brightness_set+0x82/0xf0 [rtl8187]
 [<c0867f7c>] led_trigger_event+0x5c/0x80
 [<f7ff5e6d>] ieee80211_led_radio+0x1d/0x40 [mac80211]
 [<f7ff3583>] ieee80211_stop_device+0x13/0x230 [mac80211]

Removing _sync is ok, because if led_on work is currently running
it will be finished before led_off work start to perform, since
they are always queued on the same mac80211 local->workqueue.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=795176



Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Acked-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>