[libata] sata_mv: errata check buglet fix

Fix a buglet; the errata check below this code is assuming the value in
the sstatus variable is what was pulled out of the SCR_STATUS register.
However, the status checks in the timeout loop clobber everything
but the first 4 bits of sstatus, so the errata checks are invalid.

This patch changes it to not clobber SStatus.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>