rpmsg: glink: spi: 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: If6d43ce481f28a46a344abe8eaf3ba476a3933ed
Signed-off-by: Chris Lew <clew@codeaurora.org>