EF10 functions don't have a fixed BAR size, and the minimum is not
large enough for all the queues we might want to allocate.  We have to
find out the BAR size at run-time, and therefore phys_addr_channels
and mem_map_size cannot be defined per-NIC-type.

Change efx_nic_type::mem_map_size to a function pointer which is
called to find the wanted memory map size (before probe).

Replace efx_nic_type::phys_addr_channels with efx_nic::max_channels,
to be initialised by the probe function.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>