scripts/spdxcheck.py: Handle special quotation mark comments

The SuperH boot code files use a magic format for the SPDX identifier
comment:

  LIST "SPDX-License-Identifier: .... "

The trailing quotation mark is not stripped before the token parser is
invoked and causes the scan to fail. Handle it gracefully.

Fixes: 6a0abce4 ("sh: include: convert to SPDX identifiers")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Simon Horman <horms+renesas@verge.net.au>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>