hwrng: xgene - fix handling platform_get_irq

The function can return negative value.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576



Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>