[S390] hw_random: allow rng_dev_read() to return hardware errors.

The api for hardware random number generators is currently limited to
devices that never fail. If the hardware is registered as a source for
random numbers it has to work. This prevents the use of i/o based
random number devices where the i/o might fail.

Add a check for errors after the read from a hardware random number device.

This patch is required to support large random numbers retrieved
from the CEX2C cards on System z.

Signed-off-by: Ralph Wuerthner <rwuerthn@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>