staging: ion: Use PTR_ERR_OR_ZERO

PTR_RET is deprecated. Use PTR_ERR_OR_ZERO instead.
While at it also use PTR_ERR_OR_ZERO in ion.c to simplify
the code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>