mfd: iqs62x: Do not poll during ATI

After loading firmware, the driver triggers ATI (calibration) with
the newly loaded register configuration in place. Next, the driver
polls a register field to ensure ATI completed in a timely fashion
and that the device is ready to sense.

However, communicating with the device over I2C while ATI is under-
way may induce noise in the device and cause ATI to fail. As such,
the vendor recommends not to poll the device during ATI.

To solve this problem, let the device naturally signal to the host
that ATI is complete by way of an interrupt. A completion prevents
the sub-devices from being registered until this happens.

The former logic that scaled ATI timeout and filter settling delay
is not carried forward with the new implementation, as it produces
overly conservative delays at lower clock rates. Instead, a single
pair of delays that covers all cases is used.

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>