TTY: serial_core, remove invalid test

tty->index (named here as line) is set up in initialize_tty_struct.
The value is checked in get_tty_driver for the found driver as:
	if (device < base || device >= base + p->num)
		continue;
	*index = device - base;

So index/line can never be more than driver->num. Hence remove this
test from uart_open.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>