[libata] sata_mv: Fix 50xx irq mask

IRQ mask bits assumed a 60xx or newer generation chip, which is very
wrong for the 50xx series.  Luckily both generations shared the per-port
interrupt mask bits, leaving only the "misc chip features" bits to be
completely mismatched.

Fix 50xx by ensuring we only program bits that exist.

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