rpmsg: glink: initialize ret to zero to ensure error status check is correct

The new switch cases for RPM_CMD_RX_DONE, RPM_CMD_RX_DONE_W_REUSE,
RPM_CMD_RX_INTENT_REQ_ACK, RPM_CMD_INTENT and RPM_CMD_RX_INTENT_REQ from
4 recent commits are not setting ret and so a later non-zero check on ret
is testing on a garbage value in ret. Fix this by initializing ret to zero.

Detected by CoverityScan CID#1455249 ("Uninitialized scalar variable")

Fixes: 933b45da ("rpmsg: glink: Add support for TX intents)
Fixes: dacbb35e ("glink: Receive and store the remote intent buffers")
Fixes: 27b9c5b6 ("rpmsg: glink: Request for intents when unavailable")
Fixes: 88c6060f ("rpmsg: glink: Handle remote rx done command")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>