[media] rtl28xxu: fix control message flaws

Add lock to prevent concurrent access for control message as control
message function uses shared buffer. Without the lock there may be
remote control polling which messes the buffer causing IO errors.
Increase buffer size and add check for maximum supported message
length.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=103391
Fixes: c56222a6

 ("[media] rtl28xxu: move usb buffers to state")

Cc: <stable@vger.kernel.org> # 4.0+
Signed-off-by: Antti Palosaari <crope@iki.fi>