rpmsg: glink: Add lock to rpmsg_ctrldev_remove

Hold ctrl device lock in rpmsg_ctrldev_remove to avoid any
new create ept call to proceed, otherwise new ept creation
and associted char device may suceed. Any further call from
user space for rpmsg_eptdev_open will reference already freed
rpdev and will result in crash. Below crash signature was
observed -

rpmsg_create_ept+0x40/0xa0
rpmsg_eptdev_open+0x88/0x138
chrdev_open+0xc4/0x1c8
do_dentry_open+0x230/0x378
vfs_open+0x3c/0x48
path_openat+0x93c/0xa78
do_filp_open+0x98/0x118
do_sys_openat2+0x90/0x220
do_sys_open+0x64/0x8c

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