media: v4l: common: Fix naming of v4l2_get_link_rate

Rename v4l2_get_link_rate() as v4l2_get_link_freq(). What the function
returns is the frequency of the link; rename it to reflect the name of the
control where the information is obtained.

Fixes: 1b888b3c ("media: v4l: Add a helper for obtaining the link frequency")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>