msm: mdss: Use correct error checking for mdss bus scale register

Previously, we were checking the error value for mdss bus_scale_register
incorrectly. Now we check the error value for null rather than a negative
value.

Change-Id: If7fda0584df82d6c007829f2e63af9b735cafb7e
Signed-off-by: Benet Clark <benetc@codeaurora.org>