dfu: mmc: fs: Fix format accepted by ext4write command

The commit:

"EXT4: Fix number base handling of "ext4write" command"
 SHA1: f7740f77

Cleaned up the ext4write command format.

This commit shall be regarded as a follow up, since the DFU subsystem is
using those commands for its normal operation.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>