scripts/spelling.txt: add misspelled words for check

misspelled words for check:-
 chcek
 chck
 cehck

I myself did these spell mistakes in changelog for patches, Thus
suggesting to add in spelling.txt, so that checkpatch.pl warns it
earlier.  References:-

./arch/powerpc/kernel/exceptions-64e.S:456: . . . make sure you chcek
https://lkml.org/lkml/2015/6/25/289


./arch/x86/mm/pageattr.c:1368: * No need to cehck in that case

[akpm@linux-foundation.org: add whcih->which, whcih I always get wrong]
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>