TTY: serial, cleanup atmel_set_ldisc

Current ldisc number is passed as a paramneter -- no need to dig it
out of the tty or ldisc. So switch PPS check to that.

No tty callback can be called with port->line higher than TTY driver
num. So remove the check.

This removes some port.tty users.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Viktar Palstsiuk <viktar.palstsiuk@promwad.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>