drm/msm/dsi-staging: alter length to avoid stepping out of bounds

Limit max length of buffer while read misr configuration
from sysfs node.

Change-Id: I9adfd51da7ae77201cf6c0cf6e9af68785c7a063
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>