powerpc: Remove arch specific byteswappers from the MXC MMC driver

When the MXC MMUC driver is used on a Freescale MPC512x machine, it
contains some additional byteswapping code (I'm assuming this is a
workaround for a hardware defect).  This uses the ppc specific st_le32()
function, but there's no reason not to use the generic swab32() function
instead.  gcc is capable of generating the efficient ppc byte-reversing
load/store instructions without the arch-specific helper.

This patch, therefore, switches to the generic byteswap routine.

Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <kernel@pengutronix.de>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>