Wait for SPU transfer

0001865c: 30840010 andi r4,r4,0x0010
00018660: 10800007 beq r4,r0,0x00018680
00018664: 00000000 nop
00018668: 3c028003 lui r2,0x8003
0001866c: 94422a54 lhu r2,0x2a54(r2)              # encoded instruction
00018670: 00000000 nop
00018674: 30420010 andi r2,r2,0x0010
00018678: 1440fffb bne r2,r0,0x00018668           # loop if transfer of some kind in progress (holds CPU until rescued by IRQ)
0001867c: 00000000 nop
00018680: 3c028003 lui r2,0x8003
00018684: 94422a54 lhu r2,0x2a54(r2)              # else
00018688: 00000000 nop
0001868c: 30420010 andi r2,r2,0x0010
00018690: 10400008 beq r2,r0,0x000186b4           # end if transfer of some kind not in progress
00018694: 00001021 addu r2,r0,r0
00018698: 3c028003 lui r2,0x8003                  # this seems only possible to reach in some kind of error.
0001869c: 94422a30 lhu r2,0x2a30(r2)
000186a0: 3c038003 lui r3,0x8003
000186a4: 8c632a04 lw r3,0x2a04(r3)
000186a8: 00021100 sll r2,r2,0x04
000186ac: 00431021 addu r2,r2,r3
000186b0: 84420000 lh r2,0x0000(r2)
000186b4: 03e00008 jr r31
000186b8: 00000000 nop