msm: vidc: Copy user buffers to kernel memory before access

trigger_ssr_write directly accesses user buffer, which can lead
to crash. Copy user buffer to kernel memory before access.
Also, set initial value for ssr_trigger_val, and use kstrtoul
instead of sscanf, with proper checking of return code.

Change-Id: I94b1c14a8ae4628cb6ac0ee7f3bd38b0c79f088b
Signed-off-by: Surajit Podder <spodder@codeaurora.org>