clk: ccf: replace the get_rate helper

The 12d15262 commit fixed the get_rate helper because the set_parent
one did not re-parent the clock device to the new parent. The 4d139f38
commit allows you to remove this workaround by calling the
clk_get_parent_rate routine.

Signed-off-by: Dario Binacchi <dariobin@libero.it>