msm: kgsl: Fixing min_pwrlevel kgsl sysfs element

The change 30a2b230 modified kgsl_sysfs_store function calls
everywhere except in the kgsl_pwrctrl_min_pwrlevel_store()
function. As a result min_pwrlevel sysfs item behaved incorrectly.

Correct the return code check for kgsl_sysfs_store call in
kgsl_pwrctrl_min_pwrlevel_store().

Change-Id: If2b0cc9b26625c1a326b6644ab183a36868666f9
Signed-off-by: Vladimir Razgulin <vrazguli@codeaurora.org>