staging: gdm72xx: clean up endianness conversions

This patch cleans up the endianness conversions in the gdm72xx driver:
- Directly use the generic byte-reordering macros for endianness
  conversion instead of some custom-defined macros.
- Convert values on the constant side instead of the variable side when
  appropriate.
- Add endianness annotations.

Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>