digsig: return -EPERM in digsig_key_store() if module initialized

Return -EPERM rather than -1 in digsig_key_store() if the module has
already been initialized (a key has already been loaded), to clarify
the meaning of the code. EPERM is defined as 1, so this patch is only
a readability improvement.

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>