cmd/usb_mass_storage.c: Rework ums_init() ret logic slightly

cmd/usb_mass_storage.c: Rework ums_init() ret logic slightly

Previously, ret could be used uninitialized if
blk_get_device_part_str() failed.  Default to ret being set to -1 so
that we always return an err up if we have a problem and then invert the
logic on testing ums_count as when that is non-zero is the time we can
return 0.

Cc: John Tobias <john.tobias.ph@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>