iwlwifi: pcie: don't increment / decrement a bool

David reported that the code I added uses the decrement
and increment operator on a boolean variable.

Fix that.

Fixes: 0cd58eaa ("iwlwifi: pcie: allow the op_mode to block the tx queues")
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>