disk: part: Don't skip partition init

blk_get_device_by_str was skipping part_init when hw partition 0 was
selected because it is the default. However, this caused issues when
switching to a non-zero partition and then back to partition zero, as
stale data from the wrong partition was returned.

Remove this optimization and call part_init regardless of the selected
partition.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>