s390/ap: function rework based on compiler warning

Slight rework of function __ap_revise_reserved()
because of unused variable warning when build with W=1.
This patch introduces an additional debug feature warning
message when device_reprobe() returns with failure.
However, the return value of __ap_revise_reserved()
is still hard coded to 0 as this is a callback function
to be used together with bus_for_each_dev() and thus
the return value indicates to go on with the
bus_for_each_dev() loop and not apport on a failure
of something within this function.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>