dfu: mmc: change offset base handling

Previously offsets handled by dfu_fill_entity_mmc(), defined in boards'
CONFIG_DFU_ALT were treated as hexadecimal regardless of their prefix,
which sometimes led to confusion. This patch forces usage of explicit
numerical base prefixes.

Signed-off-by: Mateusz Zalega <m.zalega@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Tom Rini <trini@ti.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>