net: rmnet_data: Use standard endianness macros in structs

Replace all instances of RMNET_USE_BIG_ENDIAN_STRUCTS in header
definitions with #if defined(__LITTLE_ENDIAN_BITFIELD). The replacement
macro is well known and used in the core network stack.

Change-Id: I9a174ca40e186024ee9535e1b6c458bec60d09dc
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>