The external clock frequency was set by the driver but the obtained
frequency was never verified. Do that.

Being able to obtain the exact frequency is important as the value is used
for PLL calculations which may result in frequencies that violate the PLL
tree limits.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>