staging:iio: rename gyro channels to anglvel

Ensure naming reflects what is measured, not how it is done.
Resolvers can measure the same thing for starters.
IIO_GYRO->IIO_ANGL_VEL to ensure consistent naming.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>