s390/pci: make better use of zpci_dbg() levels

While the zpci_dbg() macro offers a level parameter this is currently
largely unused. The only instance with higher importance than 3 is the
UID checking change debug message which is not actually more important
as the UID uniqueness guarantee is already exposed in sysfs so this
should rather be 3 as well.

On the other hand the "add ..." message which shows what devices are
visible at the lowest level is essential during problem determination.
By setting its level to 1, lowering the debug level can act as a filter
to only show the available functions.

On the error side the default level is set to 6 while all existing
messages are printed at level 0. This is inconsistent and means there is
no room for having messages be invisible on the default level so instead
set the default level to 3 like for errors matching the default for
debug messages.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>