soc: hab: change lifecycle of exp_id from vchan to ctx

To make sure unexport/unimport the same buffer successfully
in different threads from export/import function, only check
pchan's validation, instead of vchan id.

Change-Id: I3203f198c37e8b169090d8f93d92e87bbd4cdb6e
Signed-off-by: Yajun Li <yajunl@codeaurora.org>