[libata] sata_mv: Fix and clean up per-chip-generation tests

Due to a mistake in test logic, Gen-IIE chips were being treated as
Gen-II chips in some cases.  Fix this, and in the process, clean up
IS_50XX/IS_60XX tests to the more uniform IS_GEN_{I,II,IIE} tests.

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