msm: smd_tty: add separate port locking

Currently a single mutex is used to lock access to opening and closing
SMD TTY ports.  This lock is held while waiting for the open timeout to
expire which unnecessarily delays ports that could be opened
immediately.

Add an individual lock for each port to enable each port to be opened
and closed independently.

CRs-Fixed: 541915
Change-Id: Ic698d9b6a1e5f6f4676a6530d3c7c0bc52ba35aa
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>