net/sfc/base: add background mode firmware updating

net/sfc/base: add background mode firmware updating

Request firmware updates be performed in background mode.
In this mode MCDI to the function processing the update
remains accessible and the client polls for completion.
This is supported for lengthy partition updates such as
MCFW and bundles. The MC ignores the flags used for this
mode for other partition updates.

Signed-off-by: Richard Houldsworth <rhouldsworth@solarflare.com>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>