i40e: Remove duplicated prepare call in i40e_shutdown

Function call to i40e_prep_for_reset() is duplicated in
i40e_shutdown routine and gets called before
i40e_enable_mc_magic_wake() which blocks it from being executed
correctly on system reboot or shutdown because adminq is already
disabled by first i40e_prep_for_reset() call.

Two register write calls are also duplicated.

Signed-off-by: Sergey Nemov <sergey.nemov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>