hwrng: bcm2835 - fix probe as platform device

BCM63XX (MIPS) does not use device tree, so there cannot be any
of_device_id, causing the driver to fail on probe:

[    0.904564] bcm2835-rng: probe of bcm63xx-rng failed with error -22

Fix this by checking for match data only if we are probing from device
tree.

Fixes: 8705f24f ("hwrng: bcm2835 - Enable BCM2835 RNG to work on BCM63xx platforms")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>