carl9170: fix scheduling while atomic

This patch fixes the following mishap:

BUG: scheduling while atomic: wpa_supplicant/4164/0x00000002
Modules linked in: carl9170 mac80211 [...]
Pid: 4164, comm: wpa_supplicant Not tainted 2.6.36-wl+ #119
Call Trace:
 [<c13779a9>] ? schedule+0x349/0x4c0
 [<c13780d6>] ? schedule_timeout+0x106/0x1e0
 [<c1037f50>] ? process_timeout+0x0/0x10
 [<c1377e8d>] ? wait_for_common+0x9d/0x140
 [<c1029110>] ? default_wake_function+0x0/0x10
 [<f80c6080>] ? carl9170_exec_cmd+0xf0/0x250 [carl9170]
 [<f80c695e>] ? carl9170_set_mac_reg+0x5e/0x70 [carl9170]
 [<f80c3f76>] ? carl9170_op_add_interface+0x176/0x310 [carl9170]
 [...]

rcu_read_unlock() call was erroneously placed after the
sync. function carl9170_mod_virtual_mac.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>