ENGR00221975 Fix race condition in clock code.

ENGR00221975 Fix race condition in clock code.

Need to ensure that check for usecount in clk_set_parent
occurs within the protection of the clock mutex. Else
there is a chance that the usecount can be decremented
(and the clock disabled) after the check.
Also add back the code to maintain the correct usecount
for pll2_pfd_400.

Signed-off-by: Ranjani Vaidyanathan <ra5478@freescale.com>