[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 <jgarzik@redhat.com>