mtd: cmdlinepart: revise error handling

This patch revises and fixes error handling in the command line mtd
partitions parser. Namely:
1. we ignored return code of 'mtdpart_setup_real()'.
2. instead of returning 0 for failure and 1 for success, teach
   'mtdpart_setup_real()' to return real error codes.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>