Fix read_ivar implementation for MMC and SD.

Fix read_ivar implementation for MMC and SD.

1. Both mmc_read_ivar() and sdhci_read_ivar() use the expression
'*(int *)result = val' to assign to result which is uintptr_t *.
This does not work on big-endian 64 bit systems.

2. The media_size ivar is declared as 'off_t' which does not fit
into uintptr_t in 32bit systems, change this to long.

Submitted by:	kanthms at netlogicmicro com (initial version)