powerpc/kerenl: Enable EEH for IO accessors

In arch/powerpc/kernel/iomap.c, lots of IO reading accessors missed
to check EEH error as Ben pointed. The patch fixes it.

For the writing accessors, we change the called functions only for
making them look similar to the reading counterparts.

Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>