s390/pci: clean up left over special treatment for function zero

s390/pci: clean up left over special treatment for function zero

Prior to commit 960ac3626487 ("s390/pci: allow zPCI zbus without
a function zero") enabling and scanning a PCI function had to
potentially be postponed until the function with devfn zero on that bus
was plugged. While the commit removed the waiting itself extra code to
scan all functions on the PCI bus once function zero appeared was
missed. Remove that code and the outdated comments about waiting for
function zero.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/20230306151014.60913-5-schnelle@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>