iavf: Use printf instead of gnu_printf for iavf_debug_d

Clang warns:

In file included from drivers/net/ethernet/intel/iavf/iavf_main.c:4:
In file included from drivers/net/ethernet/intel/iavf/iavf.h:37:
In file included from drivers/net/ethernet/intel/iavf/iavf_type.h:8:
drivers/net/ethernet/intel/iavf/iavf_osdep.h:49:18: warning: 'format' attribute argument not supported: gnu_printf [-Wignored-attributes]
        __attribute__ ((format(gnu_printf, 3, 4)));
                        ^
1 warning generated.

We can convert from gnu_printf to printf without any side effects for
two reasons:

1. All iavf_debug instances use standard printf formats, as pointed out
   by Miguel Ojeda at the below link, meaning gnu_printf is not strictly
   required.

2. However, GCC has aliased printf to gnu_printf on Linux since at least
   2010 based on git history.

   From gcc/c-family/c-format.c:

   /* Attributes such as "printf" are equivalent to those such as
      "gnu_printf" unless this is overridden by a target.  */
   static const target_ovr_attr gnu_target_overrides_format_attributes[] =
   {
     { "gnu_printf",   "printf" },
     { "gnu_scanf",    "scanf" },
     { "gnu_strftime", "strftime" },
     { "gnu_strfmon",  "strfmon" },
     { NULL,           NULL }
   };

The mentioned override only happens on Windows (mingw32). Changing from
gnu_printf to printf is a no-op for GCC and stops Clang from warning.

Link: ClangBuiltLinux#111
Suggested-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

drivers/net/ethernet/intel/iavf/iavf_osdep.h

@@ -46,7 +46,7 @@ struct iavf_virt_mem {
 
 #define iavf_debug(h, m, s, ...)  iavf_debug_d(h, m, s, ##__VA_ARGS__)
 extern void iavf_debug_d(void *hw, u32 mask, char *fmt_str, ...)
-	__attribute__ ((format(gnu_printf, 3, 4)));
+	__printf(3, 4);
 
 typedef enum iavf_status_code iavf_status;
 #endif /* _IAVF_OSDEP_H_ */