rpmsg-kdrv: rpmsg_kdrv_demo_get_data() can be static

rpmsg_kdrv_demo_get_data() and related functions and
data structures can be static. Fix it.

Fixes: c93833f5 ("rpmsg-kdrv: add access APIs from virtual drivers")
Signed-off-by: kernel test robot <lkp@intel.com>
[nsekhar@ti.com: add a description]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>