msm: rmnet_ipa: add support for RMNET_IOCTL_GET_SG_SUPPORT

While the transport always supports scatter-gather, the
capability advertised through this IOCTL is configured
from the device tree.

Change-Id: Ied6a004c57346d1324b600991f069363d8a2f48f
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>