net/mlx5: Reduce kconfig complexity while building crypto support

Both IPsec and kTLS need two functions declared in the lib/crypto.c
file. These functions are advertised through general mlx5.h file and
don't have any protection from attempts to call them without proper
config option.

Instead of creating stubs just for two functions, simply build that *.c
file as part of regular mlx5_eth build and rely on compiler to throw
them away if no callers exist in produced code.

Link: https://lore.kernel.org/r/37f02171da06886c1b403d44dd18b2a56b19219d.1649232994.git.leonro@nvidia.com
Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>