crypto: ecdh - remove empty exit()

Pointer members of an object with static storage duration, if not
explicitly initialized, will be initialized to a NULL pointer. The crypto
API checks if this pointer is not NULL before using it, we are safe to
remove the function.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>