s390/crypto: avoid returning garbage value

Static analysis with cppcheck detected that ret is not initialized
and hence garbage is potentially being returned in the case where
prng_data->ppnows.reseed_counter <= prng_reseed_limit.

Thanks to Martin Schwidefsky for spotting a mistake in my original
fix.

Fixes: 0177db01 ("s390/crypto: simplify return code handling")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>