hwrng: cavium - make two functions static

Functions cavium_rng_remove and cavium_rng_remove_vf are local to the
source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
drivers/char/hw_random/cavium-rng-vf.c:80:7: warning: symbol
'cavium_rng_remove_vf' was not declared. Should it be static?
drivers/char/hw_random/cavium-rng.c:65:7: warning: symbol
'cavium_rng_remove' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>