scripts/spelling.txt: add "swithc" pattern and fix typo instances

Fix typos and add the following to the scripts/spelling.txt:

  swithc||switch
  swithced||switched
  swithcing||switching

Link: http://lkml.kernel.org/r/1481573103-11329-3-git-send-email-yamada.masahiro@socionext.com


Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>