[SERIAL] Pass around serial_private instead of pci_dev

Pass the serial_private structure via the setup method instead of
the pci_dev.  We don't want to assume that the pci_dev's driver
data is a pointer to serial_private.  Instead, put the pci_dev
inside serial_private.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>