net: usb: rmnet_usb_ctrl:Make sure list_head operate atomically

Get and delete operation on variables "list_elem" are not atomic.
Multiple threads may get the same "list_elem", may lead to race
conditions.

Add mutex in rmnet_ctl_open to resolve current potential race condition
between test_bit and set_bit.

Change-Id: I00c4e2fd4854ee17a13a0757da98c46a78eee4cb
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>