iio: magnetometer: ak8975: fix wrong errno on return

iio: magnetometer: ak8975: fix wrong errno on return

The driver currently returns -EINVAL on polling timeout instead of
-ETIMEDOUT.

Replace return code for -ETIMEDOUT and remove unnecessary error
message as -ETIMEDOUT is a standard POSIX error. Also replace
instances of -EINVAL in comments.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>