sfc: Move MTD operations into efx_nic_type

Merge the per-NIC-type MTD probe selection and struct efx_mtd_ops into
struct efx_nic_type.  Move the implementations into the appropriate
source files.

Several NVRAM functions are now only called from MTD operations which
are now implemented in the same file (falcon.c or mcdi.c).  There is no
need for them to be extern, or to be defined at all if CONFIG_SFC_MTD
is not enabled, so move them into the #ifdef CONFIG_SFC_MTD sections
in those files.

Most of the SPI-related definitions are also only used in falcon.c,
so move them there.  Put the remainder of spi.h into nic.h (which
previously included it).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>