aic7xxx/Makefile fix

Patch against 2.4.19. If you are in a (good) habit of making all the upstream sources read-only before starting your build, generation of the firmware code fails because it tries to write into read-only files. This bites only in configurations where CONFIG_AIC7XXX_BUILD_FIRMWARE is set to 'y'.

@@ -39,6 +39,7 @@
 $(obj-aic7xxx): aic7xxx_reg.h
 
 aic7xxx_seq.h aic7xxx_reg.h: aic7xxx.seq aic7xxx.reg aicasm/aicasm
+	rm -f aic7xxx_seq.h aic7xxx_reg.h
 	aicasm/aicasm -I. -r aic7xxx_reg.h -o aic7xxx_seq.h aic7xxx.seq
 endif