serial: 8250_pci: Make PCI class test non fatal

As has been reported the National Instruments serial cards have broken
PCI class.

The commit 7d8905d0

  ("serial: 8250_pci: Enable device after we check black list")

made the PCI class check mandatory for the case when device is listed in
a quirk list.

Make PCI class test non fatal to allow broken card be enumerated.

Fixes: 7d8905d0

 ("serial: 8250_pci: Enable device after we check black list")
Cc: stable <stable@vger.kernel.org>
Reported-by: Guan Yung Tseng <guan.yung.tseng@ni.com>
Tested-by: Guan Yung Tseng <guan.yung.tseng@ni.com>
Tested-by: KHUENY.Gerhard <Gerhard.KHUENY@bachmann.info>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>