hsr: fix a NULL pointer deref in hsr_dev_xmit()

hsr_port_get_hsr() could return NULL and kernel
could crash:

 BUG: kernel NULL pointer dereference, address: 0000000000000010
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 8000000074b84067 P4D 8000000074b84067 PUD 7057d067 PMD 0
 Oops: 0000 [#1] SMP PTI
 CPU: 0 PID: 754 Comm: a.out Not tainted 5.2.0-rc6+ #718
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014
 RIP: 0010:hsr_dev_xmit+0x20/0x31
 Code: 48 8b 1b eb e0 5b 5d 41 5c c3 66 66 66 66 90 55 48 89 fd 48 8d be 40 0b 00 00 be 04 00 00 00 e8 ee f2 ff ff 48 89 ef 48 89 c6 <48> 8b 40 10 48 89 45 10 e8 6c 1b 00 00 31 c0 5d c3 66 66 66 66 90
 RSP: 0018:ffffb5b400003c48 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: ffff9821b4509a88 RCX: 0000000000000000
 RDX: ffff9821b4509a88 RSI: 0000000000000000 RDI: ffff9821bc3fc7c0
 RBP: ffff9821bc3fc7c0 R08: 0000000000000000 R09: 00000000000c2019
 R10: 0000000000000000 R11: 0000000000000002 R...