crypto: caam - staticize caam_get_era()

caam_get_era() is only used locally, so do not export this function
and make it static instead.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>