media: pvrusb2: remove redundant NULL check

Pointer dip->stream cannot be NULL due to a shift, thus remove redundant
NULL check.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: c74e0062 ("V4L/DVB (5059): Pvrusb2: Be smarter about mode restoration")
Signed-off-by: Daniil Dulov <d.dulov@aladdin.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>