sata_sil24: configure max read request size to 4k

Due to request posting limitations, bandwidth of sil3132 is limited to
around 120MB/s with the minimum pci-e payload size (128bytes) which is
used by most consumer systems.  However, write throughput can be
slightly (~3%) increased by increasing the max read requeset size.
Configure it to 4k which is the maximum supported.  This optimization
is also done by SIMG's windows driver.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>