msm_serial: Use relative resources for iomem

Device tree iomem resources are only accessible by index, and not by
name.  The msm_serial devices always have either 1 or 2 iomem
resources, that are always in the same order.  Convert the
platform_get_resource_byname into just platform_get_resource to
facilitate device tree conversion.

Change-Id: I4fd0f1037e07f2725a2a25c7b07dea2ca9397db7
Signed-off-by: David Brown <davidb@codeaurora.org>