i2c: bcm2835: Ensure clock exists when probing

Probe function fails to recognize that upstream clock actually
doesn't yet exist because clock driver has not been initialized.
Actually try to go get the clock and test for its existence
before trying to set up a downstream clock based upon it.

This fixes a bug that causes the i2c driver not to work with
monolithic kernels.

Fixes: bebff81f ("i2c: bcm2835: Model Divider in CCF")
Signed-off-by: Annaliese McDermond <nh6z@nh6z.net>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>