staging:iio:hmc5843: Implement timeout in read function

avoid polling data ready bit forever; msleep() may be too long
for high sampling frequencies but the driver interface does not
support buffering

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Shubhrajyoti Datta <shubhrajyoti@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>