max517.c: Fix client obtainment by using iio_dev_get_devdata()

This patch uses dev_get_drvdata() and iio_dev_get_devdata() instead of
to_i2c_client() (broken!) to obtain i2c_client data.

Further, some minor typo fixes are included.
Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>