[mips] convert over =v to =r for the inline assembly.

Later gcc and clang have deprecated =v (which maps to a specific temp
register) and instead we should just use =r to have the assembler
(hopefully!) save/restore things appropriately after choosing
a register.

Tested:

• AR9344 SoC, with userreg support
• AR9331 SoC, with no userreg support

Sponsored by: Sponsored by: DARPA, AFRL (MIPS TLS user register work)