ata: libata-sff: Fix compilation warning in ata_sff_lost_interrupt()

When returning false, ata_sff_altstatus() does not return any status
value, resulting in a compilation warning in ata_sff_lost_interrupt()
("uninitialized symbol 'status'"). Fix this by initializing the local
variable "status" to 0.

Fixes: 03c0e84f ("ata: libata-sff: refactor ata_sff_altstatus()")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>