ptrace: cleanup arch_ptrace() on xtensa

Use new 'datap' variable in order to remove unnecessary castings.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>