V4L/DVB: saa7164: move dereference under NULL check

The original code dereferenced "port" before checking it for NULL.  I
moved the test down below the check.  Also I changed the comparisons a
little so people wouldn't get confused and think "port" and "buf" were
ints instead of pointers.  (Probably that's what lead to this issue in
the first place.)

There is only one caller for this function and it passes non-NULL
pointers, so this is essentially a cleanup rather than a bugfix.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>