cmd: usb_mass_storage: Use part_get_info_by_dev_and_name_or_num

This allows specifying partitions using more extended syntax. This is
particularly useful to access eMMC hardware partitions. For example,
this allows something like

	ums 0 mmc 0.0,0.1,0.2,0.3

to expose four LUNs for each of the four default eMMC hardware
partitions. Note that the comma syntax was already present, and this
syntax is already documented.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>