qtnfmac: wait for FW load work to finish at PCIe remove

Waiting for "completion" to be set in FW load thread can not be used
in case PCIe remove is called before FW load work was scheduled.
Just wait for work completion instead to avoid problems.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>