sfxge(4): rename API to check Rx scale and hash support

Rename efx_rx_scale_support_get() to efx_rx_scale_default_support_get(),
and efx_rx_hash_support_get() to efx_rx_hash_default_support_get().

All these really report is whether an exclusive RSS context was
successfully acquired at efx_rx_init().

efx_rx_scale_support_get() sounds like it reports whether the device
supports RSS, and whether exclusive or shared contexts are supported,
but it doesn't do that. Renaming it to
efx_rx_scale_default_support_get() helps to reflect that it reports
what RSS support the client gets without trying to allocate RSS
contexts itself.

Also rename efx_rx_scale_support_t to efx_rx_scale_context_type_t, to
make the enum more suitable for specifying the type of an RSS context
to be allocated.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18078