sfc: Refactor EF10 stat mask code to allow for more conditional stats

Previously, efx_ef10_stat_mask returned a static const unsigned long[], which
meant that each possible mask had to be declared statically with
STAT_MASK_BITMAP.  Since adding a condition would double the size of the
decision tree, we now create the bitmask dynamically.

To do this, we have two functions efx_ef10_raw_stat_mask, which returns a u64,
and efx_ef10_get_stat_mask, which fills in an unsigned long * argument.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>