hwrng: imx-rngc - Moving IRQ handler registering after imx_rngc_irq_mask_clear()

Issue:
While servicing interrupt, if the IRQ happens to be because of a SEED_DONE
due to a previous boot stage, you end up completing the completion
prematurely, hence causing kernel to crash while booting.

Fix:
Moving IRQ handler registering after imx_rngc_irq_mask_clear()

Fixes: 1d544944 (hwrng: mx-rngc - add a driver for Freescale RNGC)
Signed-off-by: Kshitiz Varshney <kshitiz.varshney@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>