rpmsg: glink: Use cpu_mask variable from glink structure

Use cpu_mask from glink structure instead of using local variable
while setting the cpu affinity as kernel will save cpu_mask address
and use later. Saving local variable address may lead to stale
pointer access.

Change-Id: Icfc3218edc518f3b85447051abd67b15e617ace6
Signed-off-by: Arun Prakash <app@codeaurora.org>