devfreq: bw_mon: check for the return value of start_monitor

The BWMON governor start is returning success on GOV_START event
without checking for the return value of start_monitor.
The return value of start_monitor is not being returned to
ret variable. This would cause the governor to start successfully
even when the monitor failed to start causing a NULL pointer
derefence when accessing the device attributes. Fix it by checking
the return value of start_monitor.

Change-Id: I8c1f6933d44ae4533c6b81ccda8a5c4c0da3779c
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>