gsmld_poll() currently fails to handle the following corner cases correctly:
- remote party closed the associated tty

Add the missing checks and map those to EPOLLHUP.
Reorder the checks to group them by their reaction.

Fixes: e1eaea46 ("tty: n_gsm line discipline")
Signed-off-by: NDaniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220707113223.3685-4-daniel.starke@siemens.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>