sata_sis: Support for PATA supports

This is quick rework of the patch Uwe proposed but using Kconfig not
ifdefs and user selection to sort out PATA support. Instead of ifdefs and
requiring the user to select both drivers the SATA driver selects the
PATA one.

For neatness I've also moved the extern into the function that uses it.

Signed-off-by: Alan Cox
Signed-off-by: Jeff Garzik <jeff@garzik.org>