crypto: fips - annotate fips_enable() with __init to free init memory after boot

The fips_enable() function is only invoked early during kernel boot via the
__setup() macro ("fips=" command line parameter), and is never used again
after initialization completes.

Annotating it with __init places the function in the .init.text section,
allowing the kernel to free its memory after init (when freeing_initmem()
runs), reducing runtime memory footprint.

This is a standard practice for setup/early-parse functions and has no
functional impact — the parsing logic, return values, and fips mode
setting behavior remain unchanged.

Signed-off-by: Can Peng <pengcan@kylinos.cn>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>