sfc: move modparam 'interrupt_mode' out of common channel code

EF100 only supports MSI-X, so there's no need for the new driver to
 expose this old module parameter.
Since it's now visible to the linker, we have to rename it internally
 to efx_interrupt_mode to avoid symbol collisions in non-modular
 builds.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>