rtlwifi: Increase the RX queue length for USB drivers

The current number of RX buffers queued is 32, which is too small under
heavy load. That number is doubled.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>