tty: n_smux: Fix scheduling while atomic issue for ldisc open/close

Unlock spinlock when calling platform_device functions during line
discipline (ldisc) loading an unloading.

This fixes a potential scheduling while atomic condition if the platform
device code needs to allocate memory.

Change-Id: I7dbf15dc62b2276393272392a038c1d846ea2419
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>