media: usb: dvb-usb-v2: dvb_usb_core: remove redundant code in dvb_usb_fe_sleep

Check on return value and goto instruction is redundant as the code
that follows is the goto label err.

Addresses-Coverity-ID: 1268783

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>