cmd: part: number: return hexadecimal value

At this point we are using part number sub-command to retrieve UUID
of the partition using it's name.

e.g.:
 part number mmc $mmcdev system_a system_a_index
 part uuid mmc $mmcdev:${system_a_index} system_a_uuid

Since 'part uuid' sub-command expects partition index in hex format and
'part number' returns decimal value, 'part uuid' command will provide
wrong UUID or fail.

Fixes: be683756 ("cmd: part: Add 'number' sub-command")
Cc: Dirk Behme <dirk.behme@de.bosch.com>
Reported-by: Pontus Fuchs <pontus.fuchs@se.bosch.com>
Signed-off-by: Roman Stratiienko <roman.stratiienko@globallogic.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>