sfc: efx_default_channel_type APIs can be static

This means we can remove them from efx_channel.h and avoid
naming conflicts later.
efx_channel_dummy_op_void() cannot be static as it is
used in ef100_nic.c.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>