mfd: vexpress: Handle pending config transactions

The config transactions "scheduler" was hopelessly broken,
repeating completed transaction instead of picking up
next pending one.

Fixed now. Also improved debug messages.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Reviewed-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>