scripts/spdxcheck.py: work with current HEAD LICENSES/ directory

Depending on how old your -next tree is, it may not have a master that
has the LICENSES directory.

Change the lookup to HEAD and find whatever LICENSE directory files are
used in that branch.

Miscellanea:

 - Remove the checkpatch test as it will have its own SPDX license
   identifier.

Link: http://lkml.kernel.org/r/7eeefc862194930c773e662cb2152e178441d3b8.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>