serial: mfd: fix bug in serial_hsu_remove()

Medfield HSU driver deal with 4 pci devices(3 uart ports + 1 dma controller),
so in pci remove func, we need handle them differently

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>