firewire: sbp2: replace a GFP_ATOMIC allocation

sbp2_send_management_orb() is called by sbp2_login, sbp2_reconnect, and
sbp2_remove, all which are able to sleep during memory allocations.
Actually, sbp2_send_management_orb() itself is a sleeping function.

Login and remove could allocate with GFP_KERNEL but reconnect needs
GFP_NOIO to ensure progress in low memory situations.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>