[media] dvb-usb: warn if return value for USB read/write routines is not checked

the return values for dvb_usb_generic_rw() and dvb_usb_generic_write()
should be checked, as otherwise the drivers won't be doing the right
thing in the case of errors.

So, add __must_check to both declarations.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>