crypto: sun4i-ss - make unexported sun4i_ss_pm_ops static

The sun4i_ss_pm_ops is not referenced outside the driver
except via a pointer, so make it static to avoid the following
warning:

drivers/crypto/allwinner/sun4i-ss/sun4i-ss-core.c:276:25: warning: symbol 'sun4i_ss_pm_ops' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>