powerpc/traps/e500: fix misleading error output

In machine_check_e500 exception handler is a wrong indication
in case of MCSR_BUS_WBERR - so print "Write" instead of "Read".

Signed-off-by: Wladislav Wiebe <wladislav.kw@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>