i2c: bcm2835: Store pointer to bus clock

The commit bebff81f ("i2c: bcm2835: Model Divider in CCF") introduced
a NULL pointer dereference on driver unload. It seems that we can't fetch
the bus clock via devm_clk_get in bcm2835_i2c_remove. As an alternative
approach store a pointer to the bus clock in the private driver structure.

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