iio: magnetometer: ak8975: modernize polling loops with iopoll() macros

iio: magnetometer: ak8975: modernize polling loops with iopoll() macros

The driver currently uses while loops and msleep() for polling during
conversion waits.

Replace the custom polling loops with readx_poll_timeout() and
read_poll_timeout() macros from <linux/iopoll.h>. This reduces
boilerplate, standardizes timeout handling and improves overall code
readability, keeping the original timing and error behaviour. Add
<linux/time.h> for USEC_PER_MSEC macro instead of using magic numbers.

Assisted-by: Gemini:3.1-Pro
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>