UPSTREAM: PCI: rockchip: Increase the Max Credit update interval.

UPSTREAM: PCI: rockchip: Increase the Max Credit update interval.

This increases the likelihood of link state to automatically go to L1
and save some power.

The default credit update interval of 7.5 us results in the rootport
sending UpdateFC-P and UpdateFC-NP packets too often, thus resulting
in the link never going to L1, and always staying in L0/L0s. The
value 24 us was chosen after some experiments and peeking over the
PCIe bus to see that we do enter L1 substate when there is not enough
traffic on the PCIe bus.

Change-Id: I23eccba98f6fe731bcacec80349dc05bd7baf9c1
Signed-off-by: Rajat Jain <rajatja@google.com>
Acked-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
(cherry picked from git.kernel.org/cgit/linux/kernel/git/next/linux-next.git
commit 277743ef616defc870d101f9cf9d3488aba1c1b6)