DFU: Check the number of arguments and argument string strictly

When parsing the dfu_alt_info, check the number of arguments
and argument string strictly. If there is any garbage data
(which is not able to be parsed correctly) in dfu_alt_info,
that means something wrong and user may make a typo or mis-
understanding about the syntax. Since the dfu_alt_info is
used for updating the firmware, this mistake may lead to
brick the hardware.
Thus it should be checked strictly for making sure there
is no mistake.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>