crypto: sun4i_ss_prng - convert lock to _bh in sun4i_ss_prng_generate

Lockdep detects a possible deadlock in sun4i_ss_prng_generate() and
throws an "inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage" warning.
Disabling softirqs to fix this.

Fixes: b8ae5c73

 ("crypto: sun4i-ss - support the Security System PRNG")
Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>