s390: avoid always-true comparison in s390_pci_generate_fid()

Coverity points out that the comparison "fid <= ZPCI_MAX_FID"
in s390_pci_generate_fid() is always true (because fid
is 32 bits and ZPCI_MAX_FID is 0xffffffff). This isn't a
bug because the real loop termination condition is
expressed later via an "if (...) break;" inside the loop,
but it is a bit odd. Rephrase the loop to avoid the
unnecessary duplicate-but-never-true conditional.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
