tty: smux_ctl: add device attribute for open timeout

Port open call for smux is blocking. There is a 5 seconds default
timeout for the open call. The total number of retries for opening
the port makes the total wait time to be 11 minutes. This timeout
becomes an issue if QSC does not come up in time. Adding a device
attribute lets the user space control/reduce the timeout value.

CRs-Fixed: 372629
Change-Id: Ib20bf05da740c99dbb7e94b4df96d256ecadad34
Signed-off-by: Angshuman Sarkar <angshuman@codeaurora.org>