ahci: wait longer for link.

I have observed timeouts on a cubietruck.

The increase to 40ms is completely arbitrary and Works For Me(tm). I
couldn't find a good reference for how long you are supposed to wait,
although googling around it seems like tens of ms rather than single
digits is more common. I don't think there is any harm in waiting a bit
longer.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>