Blackfin: fix typo in ptrace poking

Commit c014e15a (Blackfin: convert ptrace to new memory functions)
introduced a copy & paste typo in the ptrace poke data/text handling.  The
access_process_vm() function call was telling it to read instead of write.

Signed-off-by: Jie Zhang <jie.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>