mmc: tegra: use correct accessor for misc ctrl register

The misc control register is 32bit wide, the used readw/writew
accessors only mainipulate the low 16bit of this register. It
currently doesn't matter as all the bit changed are located in
the lower half, but together with the u32 variable used to hold
the contents of the register it is seriously confusing.

Switch to 32bit accessors to avoid any future breakage.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>