tty: n_gsm: wait until channel 0 is ready

tty: n_gsm: wait until channel 0 is ready

Currently code does not wait for channel 0 open sequence to complete before
pushing data to the other channels. Also, if userland opens tty, it will
receive EL2NSYNC. Both issues result in hard to predict initialization
sequence and possible userland failures.

Fix that by waiting channel 0 open sequence to complete before attempting
opening of the other channels. Also, if tty open() is attempted while
channel 0 is opening, wait until sequence is complete before returning to
userland.

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20241228150100.100354-2-ivo.g.dimitrov.75@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>