V4L/DVB (6624): CXUSB: return control message transfer result to caller

Callers to cxusb_ctrl_msg currently do not receive any indication that their
transfer failed.  Return the true return code from dvb_usb_generic_{rw,write}.

Signed-off-by: Chris Pascoe <c.pascoe@itee.uq.edu.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>