tty-ldisc: get rid of tty_ldisc_try_get() helper function

Now that the /proc/tty/ldiscs handling doesn't play games with 'struct
ldisc' any more, the only remaining user of 'tty_ldisc_try_get()' is
'tty_ldisc_get()' (note the lack of 'try').

And we're actually much better off folding the logic directly into that
file, since the 'try' part was always about trying to get the ldisc
operations, not the ldisc itself: and making that explicit inside of
'tty_ldisc_get()' clarifies the whole semantics.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@mail.by>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>