hwrng: omap - Change RNG_CONFIG_REG to RNG_CONTROL_REG in init

omap4_rng_init() checks bit 10 of the RNG_CONFIG_REG to determine whether
the RNG is already running before performing any initiliasation. This is not
the correct register to check, as the enable bit is in RNG_CONFIG_CONTROL.
Read from RNG_CONTROL_REG instead.

Signed-off-by: Andre Wolokita <Andre.Wolokita@analog.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>