We already have tty_driver.h, so cleanup tty.h a bit by moving out
tty_driver-related function prototypes into tty_driver.h.

Note that tty.h already includes tty_driver.h.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210723103147.18250-2-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>