drivers/i2c/dw_i2c: Re-add check for empty i2c transfer list

The check was recently removed to allow callers to pass `count == 0`.
Dereferencing the `msg` array is invalid in that case, though. Linux,
where we borrowed the i2c interface from, also treats this with -EINVAL.

Change-Id: I1eec02dd3a3fcf2d477a62cc65292fca40e469d3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66341
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Tim Crawford <tcrawford@system76.com>

src/drivers/i2c/designware/dw_i2c.c

@@ -467,7 +467,7 @@ static enum cb_err dw_i2c_transfer(unsigned int bus, const struct i2c_msg *msg,
 	size_t start;
 	uint16_t addr;
 
-	if (!msg)
+	if (count == 0 || !msg)
 		return -1;
 
 	/* Break up the transfers at the differing slave address boundary. */