brcmfmac: Remove firmware-loading code duplication

brcmf_fw_request_next_item and brcmf_fw_request_done both have identical
code to complete the fw-request depending on the item-type.

This commit adds a new brcmf_fw_complete_request helper removing this code
duplication.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>