[libata] sata_sil: fix used-uninit warning

Init 'serror' to silence the following warning:

drivers/ata/sata_sil.c: In function ‘sil_interrupt’:
drivers/ata/sata_sil.c:453:14: warning: ‘serror’ may be used uninitialized in
this function [-Wuninitialized]

This is not a 'can never happen' but is nonetheless extremely unlikely.
The easiest and cleanest warning fix is simply to init the var,
rather than worry about marking the var uninit-ok.

Signed-off-by: Jeff Garzik <[e-mail]>

drivers/ata/sata_sil.c

···
 	u8 status;
 
 	if (unlikely(bmdma2 & SIL_DMA_SATA_IRQ)) {
-		u32 serror;
+		u32 serror = 0xffffffff;
 
 		/* SIEN doesn't mask SATA IRQs on some 3112s.  Those
 		 * controllers continue to assert IRQ as long as