rtlwifi: Initialize power-setting callback for USB devices

Commit a269913c entitled "rtlwifi: Rework rtl_lps_leave() and
rtl_lps_enter() to use work queue" has two bugs for USB drivers.
Firstly, the work queue in question was not initialized. Secondly,
the callback routine used by this queue is contained within the
file used for PCI devices. As a result, it is not available for
architectures without PCI hardware.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Reported-by: Richard Genoud <richard.genoud@gmail.com>
Tested-by: Richard Genoud <richard.genoud@gmail.com>
Cc: Richard Genoud <richard.genoud@gmail.com>
Cc: Stable <stable@vger.kernel.org> [3.10]
Signed-off-by: John W. Linville <linville@tuxdriver.com>