ALSA: scarlett2: Add support for Gen 4 style parameters

Writing Scarlett Gen 4 parameters differs from Gen 2 and Gen 3:
- the values are written into a shared write location
- the values are only byte-sized
- the read locations now extend beyond 0xFF
- a separate NVRAM save step is no longer required

This patch implements that alternate write style, triggered by setting
the config item size field to zero.

The write address is specified through a new config set field
gen4_write_addr.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/1624e6d8a0c629c3bdfe53825b16e8b589724fc4.1703612638.git.g@b4.vu