n_tty: Inline check_unthrottle() at lone call site

2-line function check_unthrottle() is now only called from
n_tty_read(); merge into caller.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>