ARM: dts: msm: Specify non-default data receive mtu for MSM8939

Increasing the downlink MTU to 4k shows a 14% improvement in end-to-end
bidirectional throughput when the USB link to a host PC is bandwidth
limited.

CRs-Fixed: 678204
Change-Id: I9cf5d68a14d20d6af3eaed962449b13539e009d4
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>