iio:magnetometer:ak8975: fix and enhancements

Various fixes and enhancements for the ak8975 magnetometers family driver.

Changes since v1:
* get rid of silly le16 to cpu conversion since SMBUS handles this for us when reading words (patch 3/6)
* get rid of axes caching and serialize bus access between trigger handler and read_raw
* pack triggered buffer sampling bus accesses into a single SMBUS data block access
* use channel address within read_raw

Changes since v2:
* use devm_regulator_get to allow dummy regulator usage
* ensure regulator is properly disabled
* drop first 3 patches since applied
* document in_magn_matrix sysfs attribute
* single line comments where appropriate
* remove explicit zero init for shift field of iio_chan_spec structure as that's the default
* drop unnecessary ak8975_read_axis signature modification
* remove useless cosmetic changes related to ak8975_channels
* normalize and make error handling code paths clearer
* make ak8975_remove less nitpick'able :)

Changes since v3:
* drop first patch as applied
* drop second patch as mounting matrix support will be included in another patch series (see "iio sensor mounting matrix support" thread)

Gregor.

Gregor Boirie (1):
iio:magnetometer:ak8975: triggered buffer support

 drivers/iio/magnetometer/Kconfig  |   2 +
 drivers/iio/magnetometer/ak8975.c | 135 +++++++++++++++++++++++++++++++-------
 2 files changed, 112 insertions(+), 25 deletions(-)

2.1.4