sata-fsl: Apply link speed limits

The driver was ignoring limits requested by libata.force.  The output
would look like:

fsl-sata ffe18000.sata: Sata FSL Platform/CSB Driver init
ata1: FORCE: PHY spd limit set to 1.5Gbps
ata1: SATA max UDMA/133 irq 74
ata1: Signature Update detected @ 0 msecs
ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 310)

Signed-off-by: Martin Hicks <mort@bork.org>
Signed-off-by: Tejun Heo <tj@kernel.org>