V4L/DVB (7447): pvrusb2: Fix compilation warning

Fix use of a non-int (size_t) being passed in a printf width field.
This benign issue has apparently been around for a long time, but went
undetected until now.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>