x86/efi-bgrt: Switch pr_err() to pr_debug() for invalid BGRT

It's totally legitimate, per the ACPI spec, for the firmware to
set the BGRT 'status' field to zero to indicate that the BGRT
image isn't being displayed, and we shouldn't be printing an
error message in that case because it's just noise for users. So
swap pr_err() for pr_debug().

However, Josh points that out it still makes sense to test the
validity of the upper 7 bits of the 'status' field, since
they're marked as "reserved" in the spec and must be zero. If
firmware violates this it really *is* an error.
Reported-by: Tom Yan <tom.ty89@gmail.com>
Tested-by: Tom Yan <tom.ty89@gmail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1438936621-5215-2-git-send-email-matt@codeblueprint.co.ukSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/platform/efi/efi-bgrt.c

@@ -50,8 +50,13 @@ void __init efi_bgrt_init(void)
 		       bgrt_tab->version);
 		return;
 	}
+	if (bgrt_tab->status & 0xfe) {
+		pr_err("Ignoring BGRT: reserved status bits are non-zero %u\n",
+		       bgrt_tab->status);
+		return;
+	}
 	if (bgrt_tab->status != 1) {
-		pr_err("Ignoring BGRT: invalid status %u (expected 1)\n",
+		pr_debug("Ignoring BGRT: invalid status %u (expected 1)\n",
 			 bgrt_tab->status);
 		return;
 	}