V4L/DVB (8491): stkwebcam: Always reuse last queued buffer

This change keeps the video stream going on when the application
is slow queuing buffers, instead of spamming dmesg and hanging.

Fixes a problem with aMSN reported by Samed Beyribey <beyribey@gmail.com>

Signed-off-by: Jaime Velasco Juan <jsagarribay@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>