bnxt_en: Don't cancel sp_task from bnxt_close_nic().

When implementing driver reset from tx_timeout in the next patch,
bnxt_close_nic() will be called from the sp_task workqueue.  Calling
cancel_work() on sp_task will hang the workqueue.

Instead, set a new bit BNXT_STATE_IN_SP_TASK when bnxt_sp_task() is running.
bnxt_close_nic() will wait for BNXT_STATE_IN_SP_TASK to clear before
proceeding.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>