mmc: provide a select_hwpart implementation for get_device()

This enables specifying which eMMC HW partition to target for any U-Boot
command that uses the generic get_partition() function to parse its
command-line arguments.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
v2: Propagate error code from mmc_switch_part() rather than squashing it
    to -1 in mmc_select_hwpart().
cmd_mmc: use new mmc_select_hwpart() function

The implementation of mmc_select_hwpart() was cribbed from do_mmcops().
Update do_mmcops() to call mmc_select_hwpart() to avoid duplication.

Signed-off-by: Stephen Warren <swarren@nvidia.com>