tty: fix typos/errors in tty_driver.h comments

Fix various typos and other errors in comments of tty_driver.h.  The most
significant is the wrong name of a function for the description of
TTY_DRIVER_DYNAMIC_DEV.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>