mmc: of_mmc_spi: fix little endian support

The voltage_ranges is supposed to switch from big endian to little endian.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Chris Ball <cjb@laptop.org>