Fix RGBLINK scrambling algorithm to work for SRAM

Since SRAM sections start from 0, we had been using a signed
`int8_t curScrambleSRAM` counter to allow 0 as a scrambled bank,
but this actually just placed all floating SRAM sections in bank 0.

test/link/scramble-specs/a.asm

@@ -0,0 +1,25 @@
+DEF N = 6
+
+SECTION "fixed", ROMX, BANK[3]
+; XXX: We rely on these landing at certain banks, which isn't *guaranteed*...
+FOR i, 1, N + 1
+	db BANK(xLabel{d:i})
+ENDR
+FOR i, 1, N + 1
+	db BANK(wLabel{d:i})
+ENDR
+FOR i, 1, N + 1
+	db BANK(sLabel{d:i})
+ENDR
+ds $1000 - N * 3, $ff
+
+FOR i, 1, N + 1
+	SECTION "floating{d:i}", ROMX
+	xLabel{d:i}:: ds $2000, i
+
+	SECTION "wram{d:i}", WRAMX
+	wLabel{d:i}:: dw
+
+	SECTION "sram{d:i}", SRAM
+	sLabel{d:i}:: dw
+ENDR