bnxt_en: Refactor bnxt_close_nic().

Add a new __bnxt_close_nic() function to do all the work previously done
in bnxt_close_nic() except waiting for SRIOV configuration.  The new
function will be used in the next patch as part of SRIOV cleanup.

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