sfxge(4): fix diagnostics support build without Siena

The compilation failed because __efx_sram_pattern_fns was used in
efx_nic.c, but defined in efx_sram.c which is only needed when
supporting Siena.

To fix it move all the code using __efx_sram_pattern_fns into
Siena-specific files (except for the definition in efx_sram.c itself,
as that file only needs to be included in Siena-supporting builds
anyway).

The functions to test registers and tables are unlikely to apply to any
new hardware and so can be moved into Siena files. Since Huntington
such tests have been implemented in firmware.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18117