wil6210: add support for spi slave reset

As SPI core is not being reset in SW reset, FW needs to disable it
before the reset, to ensure proper SPI re-initialization after the
reset.
Add a new WMI command to notify FW to reset SPI slave before
performing SW reset.

Change-Id: I12d23560dbb28daa64cbebec155847fe32625b63
Signed-off-by: Maya Erez <merez@codeaurora.org>