iwlwifi: mvm: re-enable TX after channel switch

The FW relies on the re-enablement of the TX in order to know
it can exit quiet mode. Currently in case of CSA with quiet mode,
the quiet mode is never cancelled, resulting in a complete traffic
hang.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008180656.fee389c83ded.I09550fdadb61f899242d7e7b7578672372e2b7fe@changeid