n_hdlc: switch tbusy and woke_up to bools

They are in fact bools, so save some bytes (8B on x86_64). Also describe
@woke_up as we know what it is.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-17-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>