drm/msm/dsi-staging: update return value to dyn clk sysfs node

While switching the dsi bit clk via sysfs node in command
mode, return value should be count of number of characters.

Change-Id: I90b3a34e2bb403257fdc8174ccbcee0e6e8ed8ef
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>