mfd: rave-sp: Emulate CMD_GET_STATUS on device that don't support it

CMD_GET_STATUS is not supported by some devices implementing
RDU2-compatible ICD as well as "legacy" devices. To account for that
fact, add code that obtains the same information (app/bootloader FW
version) using several different commands.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>