IIO: Add 4-byte unsigned reads into generic-buffer example

Add unsigned 32bit-wide reads into the generic-buffer.c

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Juergen Beisert <jbe@pengutronix.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>