ehea: Remove uses of virt_to_abs() and abs_to_virt()

abs_to_virt() simply calls __va() and we'd like to get rid of it,
so replace all abs_to_virt() uses with __va().

Similarly virt_to_abs() just calls __pa().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>