V4L/DVB (13141): gspca_sq905c: once one frame is discarded it keeps discarding all frames

While checking all gspca sub drivers pkt_scan functions for a bug I found in
1 of them (and after checking also in another), I noticed a bug in the
gspca_sq905c work queue function, once it has decided to start discarding a
frame because the application is not reading fast enough (and thus returning
buffers to fill fast enough), it never stops discarding.

This patch fixes this by simply completely removing the "discarding"
variable, if we need to discard the current frame because there is no buffer
to store it, the "frame" pointer will be NULL, so that is all we need to
check.

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