fastboot: Remove mmcpart argument from raw_part_get_info_by_name

The only thing mmcpart was used for was to pass to blk_dselect_hwpart.
This calls blk_dselect_hwpart directly from raw_part_get_info_by_name. The
error handling is dropped, but it is reintroduced in the next commit
(albeit less specificly).

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>