test: dm: fwu: fix for the updated api

fwu_get_mdata() no more requires 'dev' argument and
fwu_check_mdata_validity() has been rendered useless and dropped.
Fix the test cases to work with aforementioned changes.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Sughosh Ganu <sughosh.ganu@linaro.org>