checkpatch: fix spurious vendor compatible warnings

With a compatible string like

  compatible = "foo";

checkpatch will currently try to find "foo" in vendor-prefixes.txt,
which is wrong since the vendor prefix is empty in this specific case.

Skip the vendor test if the compatible is not like

  compatible = "vendor,something";
Signed-off-by: NFlorian Vaussard <florian.vaussard@epfl.ch>
Cc: Joe Perches <joe@perches.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2088,10 +2088,10 @@ sub process {
 					     "DT compatible string \"$compat\" appears un-documented -- check $dt_path\n" . $herecurr);
 				}
 
-				my $vendor = $compat;
 				my $vendor_path = $dt_path . "vendor-prefixes.txt";
 				next if (! -f $vendor_path);
-				$vendor =~ s/^([a-zA-Z0-9\-]+)\,.*/$1/;
+				next if $compat !~ /^([a-zA-Z0-9\-]+)\,/;
+				my $vendor = $1;
 				`grep -Eq "$vendor" $vendor_path`;
 				if ( $? >> 8 ) {
 					WARN("UNDOCUMENTED_DT_STRING",