mmc: sdhci-pxav2: Drop unused struct sdhci_pxa

struct sdhci_pxa is private data of PXA SDHCI driver, but not used in
sdhci-pxav2 at all. Drop unused references to struct sdhci_pxa.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>