checkpatch: correct check for kernel parameters doc

Adding a new kernel parameter with documentation makes checkpatch complain

  __setup appears un-documented -- check Documentation/admin-guide/kernel-parameters.rst

The list of kernel parameters has moved to a separate txt file, but
checkpatch has not been updated for this.

Make checkpatch.pl look for the documentation for new kernel parameters
in kernel-parameters.txt instead of kernel-parameters.rst.

Fixes: e52347bd ("Documentation/admin-guide: split the kernel parameter list to a separate file")
Signed-off-by: Tim Froidcoeur <tim.froidcoeur@tessares.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Joe Perches <joe@perches.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>