mmc: sdhci-iproc: fix 32bit writes for TRANSFER_MODE register

When the host controller accepts only 32bit writes, the value of the
16bit TRANSFER_MODE register, that has the same 32bit address as the
16bit COMMAND register, needs to be saved and it will be written
in a 32bit write together with the command as this will trigger the
host to send the command on the SD interface.
When sending the tuning command, TRANSFER_MODE is written and then
sdhci_set_transfer_mode reads it back to clear AUTO_CMD12 bit and
write it again resulting in wrong value to be written because the
initial write value was saved in a shadow and the read-back returned
a wrong value, from the register.
Fix sdhci_iproc_readw to return the saved value of TRANSFER_MODE
when a saved value exist.
Same fix for read of BLOCK_SIZE and BLOCK_COUNT registers, that are
saved for a different reason, although a scenario that will cause the
mentioned problem on this registers is not probable.

Fixes: b580c52d ("mmc: sdhci-iproc: add IPROC SDHCI driver")
Signed-off-by: Corneliu Doban <corneliu.doban@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>