hw_random: msm_rng: fix a null pointer dereference

This change prevents a possible null pointer
dereference in msm_rng.c by moving the function
call under if block.

Change-Id: I481b39c91e495692e575a90b392095e95eb1f367
Acked-by: Saurabh Sabnis <ssabnis@qti.qualcomm.com>
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>