mips: ralink: rt3883: soc queries and tests as functions

 Move the SoC register value queries and tests to specific functions,
 to remove repetition of logic. No functional changes intended

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>