crypto: omap-sham - Constify static attribute_group

The only usage of omap_sham_attr_group is to pass its address to
sysfs_{create,remove}_group(), which takes pointers to const struct
attribute_group. Make it const to allow the compiler to put it in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>