[libata] sata_sil: Fix compilation error with libata debugging enabled

I tried compiling 2.6.29-rc1 and 2.6.29-rc3 with libata debugging enabled
and got the following error:

  CC [M]  drivers/ata/sata_sil.o
drivers/ata/sata_sil.c: In function 'sil_fill_sg':
drivers/ata/sata_sil.c:327: error: 'pi' undeclared (first use in this function)
drivers/ata/sata_sil.c:327: error: (Each undeclared identifier is reported only once
drivers/ata/sata_sil.c:327: error: for each function it appears in.)
make[2]: *** [drivers/ata/sata_sil.o] Error 1
make[1]: *** [drivers/ata] Error 2
make: *** [drivers] Error 2

include/linux/libata.h has the following enabled:

#define ATA_DEBUG
#define ATA_VERBOSE_DEBUG
#define ATA_IRQ_TRAP

This fixes the compilation.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>