rpmsg: rpc: use the local device pointer in all file operations

The remote processor recovery process includes the deletion and
recreation of an rpmsg-rpc device. The representative rppc_device
structure is retained and reused if there are any open applications
using the exposed character device. The underlying device pointer
for a rppc_device is though deleted and recreated and can become
NULL at any point if an error recovery happens. So, switch to using
the local reference device pointer in all the fop functions for
the exposed character device.

Signed-off-by: Suman Anna <s-anna@ti.com>