rpmsg: glink: Fix default case while handling received commands

Currently if we receive a command that we still do not
support, then its simply discarded. While doing so, the
RX FIFO pointer also needs to be incremented. Fixing this.

Change-Id: I781c315ce0d0b3419c6b7e42c2c690c708bb8949
Signed-off-by: Sricharan R <sricharan@codeaurora.org>