sfc: cleanups around efx_alloc_channel

The old_channel argument is never used, so remove it.
The function is only called from elsewhere in efx_channels.c, so make
 it static and remove the declaration from the header file.

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