[media] dib9000: read16/write16 could return an error code

Both dib9000_read16_attr and dib9000_write16_attr can return an
error code. However, they currently return an u16. This produces the
following warnings on smatch:

	drivers/media/dvb-frontends/dib9000.c:262 dib9000_read16_attr() warn: signedness bug returning '(-121)'
	drivers/media/dvb-frontends/dib9000.c:321 dib9000_write16_attr() warn: signedness bug returning '(-22)'
	drivers/media/dvb-frontends/dib9000.c:353 dib9000_write16_attr() warn: signedness bug returning '(-121)'

Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>