cmd: fix return code of 'sf write' and 'sf read'

cmd: fix return code of 'sf write' and 'sf read'

If the offset or the size passed to the 'sf write' or 'sf read' command
exceeds the size of the SPI flash displaying the command usage is not
helpful. Return CMD_RET_FAILURE instead of CMD_RET_USAGE.

Use the CMD_RET_* constants instead of 0, 1, -1.

Simplify a logical expression in the final return statement.

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