wil6210: support loading dedicated image for sparrow-plus devices

Driver may be used in platforms where some use sparrow cards while
other use sparrow-plus cards, where different FW image is needed.
Add the capability to load dedicated FW image in case sparrow-plus
card is detected and fallback to default image if such does not exist.

Signed-off-by: Lazar Alexei <qca_ailizaro@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>