[media] gspca_main: Simplify read mode memory type checks

gspca_dev->memory == GSPCA_MEMORY_NO implies gspca_dev->nframes == 0,
so there is no need to check for both in dev_poll. The check in
dev_read also is more complex then needed, as dqbuf which dev_read
calls already does all necessary checks. Moreover dqbuf is holding
the proper locks while checking where as dev_read itself is not.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>