BACKPORT: tee: shm: inline tee_shm_get_id()

Now, when struct tee_shm is defined in public header,
we can inline small getter functions like this one.

Change-Id: If5f8bd999aaf180dc6f431aa26ccabd0260a47e4
Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit ef8e08d24ca84846ce639b835ebd2f15a943f42b)
Signed-off-by: Victor Chong <victor.chong@linaro.org>