MIPS: asm: spinlock: Replace "sub" instruction with "addiu"

"sub $reg, imm" is not a real MIPS instruction. The assembler can
replace that with "addi $reg, -imm". However, addi has been removed
from R6, so we replace the "sub" instruction with the "addiu" one.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>