sfc: remove max_interrupt_mode

All NICs supported by this driver are capable of MSI-X interrupts (only
 Falcon A1 wasn't, and that's now hived off into its own driver), so no
 need for a nic-type parameter.  Besides, the code that checked it was
 buggy anyway (the following assignment that checked min_interrupt_mode
 overrode it).

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