tty: n_smux: Increase inactivity timeout

Temporarily increase the Application processor inactivity timeout to
prevent a local inactivity timeout while the external modem is booting.
This is necessary to allow releasing a power-collapse ACK change without
breaking SMUX until the external modem changes have also been mainlined.

Once the external modem changes have been mainlined, then this timeout
will be reduced to 250ms to improve power consumption when idle.

Change-Id: Ic9e12c849dd53955c6ff4a1eaff0a2ec761ffa35
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>