net/wan/fsl_ucc_hdlc: fix reading of __be16 registers

When releasing the allocated muram resource, we rely on reading back
the offsets from the riptr/tiptr registers. But those registers are
__be16 (and we indeed write them using iowrite16be), so we can't just
read them back with a normal C dereference.

This is not currently a real problem, since for now the driver is
PPC32-only. But it will soon be allowed to be used on arm and arm64 as
well.

Reviewed-by: Timur Tabi <timur@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Li Yang <leoyang.li@nxp.com>