media: imx: imx8mq-mipi_csi2: Remove unneeded code

ret is constant in imx8mq_mipi_csi_pm_suspend(). This function cannot
return error. Remove the return variable. Simplify other functions which
are using this function.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>