ptrace: cleanup arch_ptrace() on MIPS

Use new 'addrp', 'datavp' and 'datalp' variables in order to remove
unnecessary castings.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>