blocklevel: smart_write: Avoid indirectly testing formal parameters

The early-exit tests write_buf, but write_buf is assigned to buf on
declaration. Test buf directly instead to avoid unnecessary indirection.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

libflash/blocklevel.c

@@ -513,7 +513,7 @@ int blocklevel_smart_write(struct blocklevel_device *bl, uint64_t pos, const voi
 	void *erase_buf;
 	int rc = 0;
 
-	if (!write_buf || !bl) {
+	if (!buf || !bl) {
 		errno = EINVAL;
 		return FLASH_ERR_PARM_ERROR;
 	}