n_hdlc: use clamp() for maxframe

It is easier to read. And use MAX_HDLC_FRAME_SIZE instead of magic
constant.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-6-jslaby@suse.cz


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