dsp_rom: skip bootucode_ax when running from entrypoint

This could cause the first branch of the bootucode procedure, which
takes its parameters from the AX registers, to run during the ROM init
sequence. Since the ROM doesn't set any of the AX registers, the values
aren't meaningful, and can cause bad DMA transfers and crashes.

docs/DSP/free_dsp_rom/dsp_rom.ds

@@ -55,7 +55,8 @@ param5:
 	jnz mainloop+#IROM_BASE
 	call wait_for_cpu_mbox+#IROM_BASE
 	lr $AR0, @CMBL
-	jmp 0x80b5
+	; skip the branch of bootucode that uses the AX registers
+	jmp bootucode_ix+#IROM_BASE
 
 wait_dma:
 	lrs		$AC0.M, @DSCR