bnxt_en: Fix RTNL lock usage on bnxt_update_link().

bnxt_update_link() is called from multiple code paths.  Most callers,
such as open, ethtool, already hold RTNL.  Only the caller bnxt_sp_task()
does not.  So it is a bug to take RTNL inside bnxt_update_link().

Fix it by removing the RTNL inside bnxt_update_link().  The function
now expects the caller to always hold RTNL.

In bnxt_sp_task(), call bnxt_rtnl_lock_sp() before calling
bnxt_update_link().  We also need to move the call to the end of
bnxt_sp_task() since it will be clearing the BNXT_STATE_IN_SP_TASK bit.

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