V4L/DVB (7503): usbvision: rename __PRETTY_FUNCTION__ occurrences

__PRETTY_FUNCTION__ has no sense in C lang context. Rename it as __func__

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>