mmc: at91_mci: Enable MMC_CAP_SDIO_IRQ only when it actually works.

According to the datasheets AT91SAM9261 does not support SDIO interrupts,
and AT91SAM9260/9263 have an erratum requiring 4bit mode while using slot
B for the interrupt to work.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Wolfgang Muees <wolfgang.mues@auerswald.de>
Cc: Andrew Victor <avictor.za@gmail.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>