cmd: simplify do_spi_flash()

CMD_RET_USAGE == -1. The special handling of this value at the end of
do_spi_flash() does not make any sense.

To avoid future confusion use the CMD_RET_* constants and simplify the
code.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>