ath11k: Fix napi related hang

Similar to the same bug in ath10k, a napi disable w/out it being enabled
will hang forever.  I believe I saw this while trying rmmod after driver
had some failure on startup.  Fix it by keeping state on whether napi is
enabled or not.

And, remove un-used napi pointer in ath11k driver base struct.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20200903195254.29379-1-greearb@candelatech.com