ptrace: cleanup arch_ptrace() and friends on Blackfin

Change signature of get/put_reg() according to the change of arch_ptrace()
and remove unnecessary castings.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>