iio: imu: adis: Refactor adis_initial_startup

All the ADIS devices perform, at the beginning, a self test to make sure
the device is in a sane state. Previously, the logic was that the self-test
was performed in adis_initial_startup() and if that failed a reset was done
and then a self-test was attempted again.

This change unifies the reset mechanism under the adis_initial_startup()
call. A HW reset will be done if  GPIO is configured, or a SW reset
otherwise. This should make sure that the chip is in a sane state for
self-test. Once the reset is done, the self-test operation will be
performed. If anything goes wrong with self-test, the driver should just
bail/error-out (i.e. no second attempt). The chip would likely not be a in
a sane state state if the self-test fails after a reset.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>