radio: iris: implement file read for iris driver

FM app is using the VIDIOC DQBUF ioctl to read the
event from iris driver. VIDIOC DQBUF ioctl needs
the v4l2buf type as V4L2_BUF_TYPE_PRIVATE and it
is Deprecated and not recommend to use in latest
kernel version.Also upstream kernel code doesn't
have the VIDIOC DQBUF as a valid ioctl.
Implement file read functionality to read the events
from iris driver.

CRs-Fixed: 2726060
Change-Id: I48ba91609a38ccccd53961b89252ef810a020deb
Signed-off-by: himta ram <hram@codeaurora.org>