bnxt_en: Fix bnxt_reset() in the slow path task.

In bnxt_sp_task(), we set a bit BNXT_STATE_IN_SP_TASK so that bnxt_close()
will synchronize and wait for bnxt_sp_task() to finish.  Some functions
in bnxt_sp_task() require us to clear BNXT_STATE_IN_SP_TASK and then
acquire rtnl_lock() to prevent race conditions.

There are some bugs related to this logic. This patch refactors the code
to have common bnxt_rtnl_lock_sp() and bnxt_rtnl_unlock_sp() to handle
the RTNL and the clearing/setting of the bit.  Multiple functions will
need the same logic.  We also need to move bnxt_reset() to the end of
bnxt_sp_task().  Functions that clear BNXT_STATE_IN_SP_TASK must be the
last functions to be called in bnxt_sp_task().  The common scheme will
handle the condition properly.

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