iio: core: added support for IIO_VAL_INT

iio: core: added support for IIO_VAL_INT

BugLink: https://bugs.launchpad.net/bugs/1676356
Added core support for IIO_VAL_INT in write_raw_get_fmt function.

Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
(cherry picked from commit f47dff323088462e7b0ac52d1ba41ce953a5ce20)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>