dfu: Fix build issue with gcc8

We get the following warning when buiding with gcc8:

	error: 'packed' attribute ignored for type 'u32_t *'
	{aka 'unsigned int *'} [-Werror=attributes]

Use UNALIGNED_GET() to access the data since we don't know the
alignment.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>