s390/pci: simplify CLP List PCI handling

Currently clp_get_state() and clp_refresh_fh() awkwardly use the
clp_list_pci() callback mechanism to find the entry for a specific FID
and update its zdev, respectively return its state.

This is both needlessly complex and means we are always going through
the entire PCI function list even if the FID has already been found.
Instead lets introduce a clp_find_pci() function to find a specific
entry and share the CLP List PCI request handling code with
clp_list_pci().

With that in place we can also easily make the function handle a simple
out parameter instead of directly altering the zdev allowing easier
access to the updated function handle by the caller.

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