iio: adc: bcm_iproc_adc: swap primary and secondary isr handler's

The third argument of devm_request_threaded_irq() is the primary
handler. It is called in hardirq context and checks whether the
interrupt is relevant to the device. If the primary handler returns
IRQ_WAKE_THREAD, the secondary handler (a.k.a. handler thread) is
scheduled to run in process context.

bcm_iproc_adc.c uses the secondary handler as the primary one
and the other way around. So this patch fixes the same, along with
re-naming the secondary handler and primary handler names properly.

Tested on the BCM9583XX iProc SoC based boards.

Fixes: 4324c97e ("iio: Add driver for Broadcom iproc-static-adc")
Reported-by: Pavel Roskin <plroskin@gmail.com>
Signed-off-by: Raveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>