drivers: modem: gsm: Define tx_lock timeout at the top

The tx_lock timeout is closely related to GSM_CMD_AT_TIMEOUT
& GSM_CMD_SETUP_TIMEOUT, and should be longer than both of
them, otherwise the gsm_ppp_stop might fail to lock the tx.

Define the timeout at the top of the driver and added a comment
to make it clear for the user.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>