rpmsg: glink: Set tail pointer to 0 at end of FIFO

When wrapping around the FIFO, the remote expects the tail pointer to
be reset to 0 on the edge case where the tail equals the FIFO length.

Change-Id: Ib0d2e7c3e9a3524a37792ec8cef3a2aac90e1bf6
Signed-off-by: Chris Lew <clew@codeaurora.org>