rpmsg: glink: spi: Fix reuseable intent leak

Reusuable intents are not freed on channel release, add handling to
free the local intents and destroy the intent idr structures in the
channel release work.

Change-Id: I4b314513737815d60eae2079c3c53828638ab372
Signed-off-by: Chris Lew <clew@codeaurora.org>