V4L/DVB (13231): pwc: Use kernel's simple_strtol()

Change own implementation of pwc_atoi() by simple_strtol(x, NULL, 10).

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>