greybus: arche-platform: Make fw_flashing_seq() return error

Make arche_platform_fw_flashing_seq() return error value, needed
later when we add clock enable support for FW flashing.

Testing Done: Tested on EVT1.5 platform.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Tested-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>