qtnfmac: simplify firmware state tracking

This patch streamlines firmware state tracking. In particular, state
QTNF_FW_STATE_FW_DNLD_DONE is removed, states QTNF_FW_STATE_RESET and
QTNF_FW_STATE_DETACHED are merged into a single state. Besides, new
state QTNF_FW_STATE_RUNNING is introduced to distinguish between
the following two cases:
- firmware load succeeded, firmware init process is ongoing
- firmware init succeeded, firmware is fully functional

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>