mmc: fix the condition for MMC version 4

Fix the problem that if we use the chip of MMC version 4 and
the capacity is smaller than 2GB or equal, the mmc->capacity is
invalid. According to the JEDEC Standard, the value of ext_csd's
capacity is valid if the value is more than 2GB.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Andy Fleming <afleming@freescale.com>