arm: mvebu: Espressobin: Use function mmc_get_op_cond() for detecting eMMC

Use function mmc_get_op_cond() instead of mmc_init() for detecting presence
of eMMC. Documentation for this function says that it could be used to
detect the presence of SD/eMMC when no card detect logic is available.

This function is also used by mx6cuboxi board for detecting presence of eMMC.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>