DC: Don't use $(VER_REV) with git

It seems unlikely that git will ever be able to produce a $(VER_REV)
of 6 characters or less (the requirement for the version field in
IP.BIN), so don't even try to use it.

backends/platform/dc/dreamcast.mk

@@ -29,7 +29,7 @@ IP.BIN : ip.txt
 
 ip.txt : $(srcdir)/backends/platform/dc/ip.txt.in
 	if [ x"$(VER_EXTRA)" = xgit ]; then \
-	  if [ -z "$(VER_REV)" ]; then ver="GIT"; else ver="r$(VER_REV)"; fi; \
+	  ver="GIT"; \
 	else ver="V$(VERSION)"; fi; \
 	if expr "$$ver" : V...... >/dev/null; then \
 	  ver="V$(VER_MAJOR).$(VER_MINOR).$(VER_PATCH)"; fi; \