mmc: sdhci-brcmstb: Fix incorrect capability

Clear incorrect SDHCI_CAN_64BIT capability on Broadcom MIPS based SoCs.
The MIPS based SoCs are using ADMA only, but the several SoCs have the
incorrect capability bit about ADMA 64-bit. The "brcm,bcm7425-sdhci" is
compatible string for MIPS based SoC.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>