ARM: debug: fix big endian operation for 8250 word mode

If the 8250 debug code is used in word mode on an big endian
host then the writes need to be change into little endian for
the bus.

Note, we have to re-convert the value back as the debug code
will inspect the value after writing it to see if a newline
has been written.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>