mmc: omap: fix broken PIO mode

After commit 26b88520 ("mmc:
omap_hsmmc: remove private DMA API implementation"), the Nokia N800
here stopped booting:

[    2.086181] Waiting for root device /dev/mmcblk0p1...
[    2.324066] Unhandled fault: imprecise external abort (0x406) at 0x00000000
[    2.331451] Internal error: : 406 [#1] ARM
[    2.335784] Modules linked in:
[    2.339050] CPU: 0    Not tainted  (3.6.0-rc3 #60)
[    2.344146] PC is at default_idle+0x28/0x30
[    2.348602] LR is at trace_hardirqs_on_caller+0x15c/0x1b0

...

This turned out to be due to memory corruption caused by long-broken
PIO code in drivers/mmc/host/omap.c.  (Previously, this driver had
been using DMA; but the above commit caused the MMC driver to fall
back to PIO mode with an unmodified Kconfig.)

The PIO code, added with the rest of the driver in commit
730c9b7e

 ("[MMC] Add OMAP MMC host
driver"), confused bytes with 16-bit words.  This bug caused memory
located after the PIO transfer buffer to be corrupted with transfers
larger than 32 bytes.  The driver also did not increment the buffer
pointer after the transfer occurred.  This bug resulted in data
corruption during any transfer larger than 64 bytes.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Chris Ball <cjb@laptop.org>