V4L/DVB (10064): mt9m111: mt9m111_get_global_gain() - unsigned >= 0 is always true

unsigned >= 0 is always true and fix formula

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>