fwk: Refactor fwk_module_get_sub_element_count

Refactor fwk_get_element_sub_element_count to return a status code
and store the actual count in a pointer. This avoids returning -ve
value due to any wrong configuration and use -ve values improperly.

Signed-off-by: Mina Aziz <mina.aziz@arm.com>