mmc: sdhci-esdhc-imx: fix reading cap_1 register value for mx6sl

When reading CAP_1 register for mx6sl, ignore bit[0-15] as it stores
CAP_2 register value which is new introduced in mx6sl.

Without this fix, the max clock for mx6sl may not be correct since
it's wrongly calculated by reading CAP_1 register.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>