media: cobalt: Use v4l2_calc_timeperframe helper

Currently, cobalt driver always returns 60fps in g_parm.
This patch uses the new v4l2_calc_timeperframe helper to
calculate the time per frame value.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>