rpmsg: glink: spi: Use mutex lock for idr_lock

Using glink structure idr_lock as spinlock and channel structure
intent_lock as mutexlock cause problem when last reference to channel
is put during cleanup by holding idr_lock spinlock.

Convert the idr_lock to mutex lock to avoid lock hierarchy issues.

CRs-Fixed: 2389162
Change-Id: I68e2e3646f80171f67f922418204052888eefa78
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>