firewire: initialize tag label to -1 in fw_xfer_alloc()

firewire: initialize tag label to -1 in fw_xfer_alloc()

Zero can be confused for a potentially valid value.
For example, if I load and unload sbp driver I get a lot of messages
like the following:

fw_tl_free: the xfer is not in the queue (tlabel=0, flag=0x0)
send: dst=0x00 tl=0x00 rt=0 tcode=0x0 pri=0x0 src=0x000
recv: dst=0x01 tl=0x21 rt=1 tcode=0x1 pri=0x0 src=0xffc0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe04464407e0
fw_tl_free() at fw_tl_free+0x18d/frame 0xfffffe0446440820
fw_xfer_unload() at fw_xfer_unload+0xca/frame 0xfffffe0446440840
fw_xferlist_remove() at fw_xferlist_remove+0x2f/frame 0xfffffe0446440870
sbp_detach() at sbp_detach+0x1e0/frame 0xfffffe04464408e0
device_detach() at device_detach+0x80/frame 0xfffffe0446440900
devclass_driver_deleted() at devclass_driver_deleted+0x6a/frame 0xfffffe0446440940
devclass_delete_driver() at devclass_delete_driver+0x7d/frame 0xfffffe0446440980
driver_module_handler() at driver_module_handler+0xff/frame 0xfffffe04464409d0
module_unload() at module_unload+0x32/frame 0xfffffe04464409f0
linker_file_unload() at linker_file_unload+0x24b/frame 0xfffffe0446440a40
kern_kldunload() at kern_kldunload+0xbc/frame 0xfffffe0446440a70
amd64_syscall() at amd64_syscall+0x314/frame 0xfffffe0446440bf0
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe0446440bf0

MFC after:	2 weeks