mod/clock: Removing the dead code from clock_set_rate

There is a piece of code in the above function which will never be
executed and nor required has been removed in this commit.

Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>
Change-Id: I6531b690c1b5163ab675cd6ef1c0b01156687cce