package/pkg-cargo.mk: correctly call $(subst ...)

Commit b6378631c2609742382984f6f7b93c1d9d2cdb78 introduced a call to the
subst make function, but did it incorrectly (followed by comma instead
of space). The result is that it expands to empty.

Fix the call.

Fixes:
http://autobuild.buildroot.net/results/8ea/8eacd455e4767cc7b1e912a3f910c940110c3be0/

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/pkg-cargo.mk

@@ -44,7 +44,7 @@ PKG_CARGO_ENV = \
 	CARGO_UNSTABLE_TARGET_APPLIES_TO_HOST="true" \
 	CARGO_TARGET_APPLIES_TO_HOST="false" \
 	CARGO_BUILD_TARGET="$(RUSTC_TARGET_NAME)" \
-	CARGO_TARGET_$(subst,-,_,$(RUSTC_TARGET_NAME))_LINKER=$(notdir $(TARGET_CROSS))gcc
+	CARGO_TARGET_$(subst -,_,$(RUSTC_TARGET_NAME))_LINKER=$(notdir $(TARGET_CROSS))gcc
 
 HOST_PKG_CARGO_ENV = \
 	$(PKG_COMMON_CARGO_ENV)