By Coverity check,  the clk_set_rate function dereferences the clk pointer
without checking whether it is NULL. This may cause problem when clk is NULL.
Fix the problem by adding NULL check.

Signed-off-by: Ye.Li <B37916@freescale.com>
(cherry picked from commit ded2f2958d568411274eeecd265fcc1181638335)
(cherry picked from commit b58ac1502391d87470918ff8d172b2b8dc1e7a68)