[media] dvb_usb_v2: fix pid_filter callback error logging

Code block braces were missing which leds broken error logging and compiler warning.

drivers/media/usb/dvb-usb-v2/dvb_usb_core.c: In function 'dvb_usb_ctrl_feed':
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:291:12: warning: 'ret' may be used uninitialized in this function [-Wuninitialized]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Milan Tuma <milan.olin@seznam.cz>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>