crypto: hisilicon - avoid unused function warning

The only caller of hisi_zip_vf_q_assign() is hidden in an #ifdef,
so the function causes a warning when CONFIG_PCI_IOV is disabled:

drivers/crypto/hisilicon/zip/zip_main.c:740:12: error: unused function 'hisi_zip_vf_q_assign' [-Werror,-Wunused-function]

Replace the #ifdef with an IS_ENABLED() check that leads to the
function being dropped based on the configuration.

Fixes: 79e09f30

 ("crypto: hisilicon - add SRIOV support for ZIP")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>