staging:iio:hmc5843: Split hmc5843.c to multiple files

This patch splits hmc5843.c to multiple files - the interface-agnostic
hmc5843_core.c, i2c specific hmc5843_i2c.c and header file hmc5843.h. This is
another step to add support of SPI-enabled hmc5983.

Signed-off-by: Josef Gajdusek <atx@atx.name>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>