[media] cxusb: Use a dma capable buffer also for reading

Commit 17ce039b ("[media] cxusb: don't do DMA on stack")
added a kmalloc'ed bounce buffer for writes, but missed to do the same
for reads. As the read only happens after the write is finished, we can
reuse the same buffer.

As dvb_usb_generic_rw handles a read length of 0 by itself, avoid calling
it using the dvb_usb_generic_read wrapper function.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>