staging: iio: isl29028: remove chip test and defaults from isl29028_chip_init()

isl29028_chip_init() contains the device driver defaults and
two I2C calls that detect the presence of the chip. This patch
moves these into isl29028_probe() so that this function can
be used by the power management runtinme in a followup patch. This
patch also renames isl29028_chip_init() to
isl29028_chip_init_and_power_on().

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>