V4L/DVB: tvp514x: do NOT change the std as a side effect

Several calls (try_fmt, g_parm among others) changed the current standard
as a side effect of that call. But the standard may only be changed by s_std.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Vaibhav Hiremath <hvaibhav@ti.com>
Tested-by: Vaibhav Hiremath <hvaibhav@ti.com>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>