net: rmnet_data: Increase max mux IDs to 256

Support the full range of mux IDs for special channels which have large
ID numbers. Expected runtime memory impact is an increase of ~1.5KB per
attached physical network device.

Change-Id: Ic9db497708064c31fe7ed588a855311b50d55e19
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>