serial: 8250_pci: Enable device after we check black list

If the board we are guessing has been listed in black list we don't need
to enable it twice. The associated driver, if any, will take care about
proper initialization.

To achieve this we split out two helper functions, i.e.
serial_pci_is_class_communication() and serial_pci_is_blacklisted() which will
be called before pcim_enable_device(). We can do this since PCI specification
requires class, device and vendor ID registers to be always present in the
configuration space.

As an example what happens before this patch applied
(These are some debug prints, don't search for them in kernel sources):

	serial 0000:00:04.1: Mapped GSI28 to IRQ28
	serial 0000:00:04.2: Mapped GSI29 to IRQ29
	serial 0000:00:04.3: Mapped GSI54 to IRQ54
	8250_mid 0000:00:04.1: Mapped GSI28 to IRQ28
	8250_mid 0000:00:04.2: Mapped GSI29 to IRQ29
	8250_mid 0000:00:04.3: Mapped GSI54 to IRQ54

After we will have just last three lines out of above.

	8250_mid 0000:00:04.1: Mapped GSI28 to IRQ28
	8250_mid 0000:00:04.2: Mapped GSI29 to IRQ29
	8250_mid 0000:00:04.3: Mapped GSI54 to IRQ54

While here, correct a value of error code mentioned in the comment of
serial_pci_guess_board().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>