net: enetc: extract enetc_int_vector_init/destroy() from enetc_alloc_msix()

Extract enetc_int_vector_init() and enetc_int_vector_destroy() from
enetc_alloc_msix() so that the code is more concise and readable.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>