intel/{skylake,apollolake}: Enable signalling of error condition

Testing for "devfn < 0" on an unsigned doesn't work, and i2c_bus_to_devfn
returns an int (with -1 for "error"), so use int for devfn.

Adapt Change-Id I7d1cdb6af4140f7dc322141c0c018d8418627434 to fix more
instances.

Change-Id: I001a9b484a68e018798a65c0fae11f8df7d9f564
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Found-by: Coverity Scan #1357450, #1357449
Reviewed-on: https://review.coreboot.org/17054
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

src/soc/intel/apollolake/i2c_early.c

@@ -32,7 +32,7 @@ static int i2c_early_init_bus(unsigned bus)
 	const struct lpss_i2c_speed_config *sptr;
 	enum i2c_speed speed;
 	pci_devfn_t dev;
-	unsigned devfn;
+	int devfn;
 	uintptr_t base;
 	uint32_t value;
 	void *reg;

src/soc/intel/skylake/i2c.c

@@ -24,7 +24,7 @@
 
 uintptr_t lpss_i2c_base_address(unsigned bus)
 {
-	unsigned devfn;
+	int devfn;
 	struct device *dev;
 	struct resource *res;