mmc: sdhci-of-at91: remove bogus MMC_SDHCI_IO_ACCESSORS select

I see no override of read/write callbacks in sdhci-of-at91.c.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>