serial: ucc_uart: access __be32 field using be32_to_cpu

The buf member of struct qe_bd is a __be32, so to make this work on
little-endian hosts, use be32_to_cpu when reading it.

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