rpmsg : glink: validate head and tail index before fifo read write

We are not validating head and tail index of tx and rx fifo
before using to read or write fifo. This can result in out of
bound memory access if head and tail have incorrect values.

This patch adds check for validation of head and tail index.

CRs-Fixed: 2398099
Change-Id: Ia8725a731cc7a45f7e13b09e1e62842ff44d53f3
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>