wil6210: fix check for sparrow D0 FW file

Driver fails to load FW for sparrow D0 devices in some cases.
Fix this by returning correct value from wil_fw_verify_file_exists
when D0 FW file is not detected for any reason.

Signed-off-by: Hamad Kadmany <qca_hkadmany@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>