hwrng: xgene - removed unneeded call to platform_set_drvdata()

This function call was found to be unnecessary as there is no equivalent
platform_get_drvdata() call to access the private data of the driver. Also,
the private data is defined in this driver, so there is no risk of it being
accessed outside of this driver file.

Signed-off-by: Andrei Coardos <aboutphysycs@gmail.com>
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Alexandru Ardelean <alex@shruggie.ro>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>