ptrace: cleanup arch_ptrace() on m68k

Use new 'regno', 'datap' variables in order to remove duplicated
expressions and unnecessary castings.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>