rpmsg: glink: Add lock to avoid race when rpmsg device is released

When remote host goes down glink char device channel is freed,
At the same time user space apps can still try to open/poll rpmsg
char device which will result in calling rpmsg_create_ept. This may
cause reference to already freed context of glink chardev channel and
result in below crash signatures -

1)
rpmsg_create_ept+0x40/0xa0
rpmsg_eptdev_open+0x88/0x138
chrdev_open+0xc4/0x1c8
do_dentry_open+0x230/0x378

2)
rpmsg_poll+0x5c/0x80
rpmsg_eptdev_poll+0x84/0xa4
do_sys_poll+0x22c/0x5c8

This patch adds proper lock and check condition to avoid such crash.

Signed-off-by: Deepak Kumar Singh <quic_deesin@quicinc.com>