iio: magnetometer: ak8974: Break out measurement

This breaks out the measurement code to its own function
so we can handle this without swirling it up with the
big switch() statement inside ak8974_read_raw().

Keep a local s16 helper variable for the signed value
coming out of the measurement before assigning it to the
integer *val. The local variable makes the code easier
to read and the compiler will optimize it if possible.

Cc: Nick Reitemeyer <nick.reitemeyer@web.de>
Cc: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>