net: ipa: retry TX channel stop commands

For RX channels we issue a stop command more than once if necessary
to allow it to stop.  It turns out that TX channels could also
require retries.

Retry channel stop commands if necessary regardless of the channel
direction.  Rename the symbol defining the retry count so it's not
RX-specific.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>