hwrng: cavium - fix NULL but dereferenced coccicheck error

Fix following coccicheck warning:
./drivers/char/hw_random/cavium-rng-vf.c:182:17-20: ERROR:
pdev is NULL but dereferenced.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>