n_hdlc: add helper for buffers allocation

Given both rx and tx allocations do the same, add a new helper
(n_hdlc_alloc_buf) and use it for both of them. This cleans up
n_hdlc_alloc slightly.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-15-jslaby@suse.cz


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