PM / devfreq: Fix the return value check in memlat governor

The return value of kstrtouint is erroneously checked while setting
the tunables for mem_latency governor due to which the tunables
cannot be changed from their default values.
This change rectifies that behavior.

Change-Id: Ief7dda4638ede2c97b26229f1188a1559b238920
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>