checkpatch: check vendor compatible with dashes

The current vendor compatible check will not match vendors with dashes,
like:

  compatible="asahi-kasei"

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Reported-by: Joe Perches <joe@perches.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>