staging: android: ion: make locally used functions static

Functions ion_handle_put and ion_handle_get_by_id are only used locally
in ion.c, so they should be made static as they used to be before
9590232b ("staging/android/ion : fix a race condition in the ion driver").

Signed-off-by: Markus Böhme <markus.boehme@mailbox.org>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>