iio:adc:max1363 clear list of missing features

Remove "Control of internal reference" from the list of unimplemented
features, since as of commit a405b00e, external reference is supported if
the device has a regulator and falls back to internal if it doesn't.

While we are modifying the header, let's make it more concise and remove
a redundant filename.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>