tty: Move parts of tty_init_dev into new functions

Move the 'find-tty' and 'fast-track-open' parts of init_dev() to
separate functions.

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>