[media] videobuf2: Fix a bug in fileio emulation error handling

Various error paths in fileio_init where not setting the request-count
to 0 when unrequesting the buffers on error to init the fileio emulation.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>