dfu: fix dev_part_str for file operations

dfu: fix dev_part_str for file operations

The third_arg for a dfu alt is read as an integer and is overloaded for
different supported backends. For ext4 and fat, this third_arg
represents the partition and forms the dev part string, which should
have its partition in hex. This commit fixes dfu ext4/fat usage for
devices with ten or more partitions.

Signed-off-by: Ivan Pang <ipman@amazon.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Link: https://lore.kernel.org/r/20250611050127.38011-1-ipman@amazon.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>