Remove call to txp_set_filter() from txp_attach(). txp_set_filter() needs

Remove call to txp_set_filter() from txp_attach(). txp_set_filter() needs
the ifp, so you can't call it before doing if_alloc(). Also, there's
really no need to call it here anyway: the code I originally ported from
OpenBSD incorrectly set the station address only once at device attach
time, instead of setting in txp_init(). This meant you couldn't change
the address with ifconfig txp0 ether xx:xx:xx:xx:xx:xx. I added the
call to txp_set_filter() in txp_init() to correct this, but forgot to
remove the call from txp_attach(). Until now, it never mattered.

With this fix, the txp driver tests good:

txp0: <3Com 3cR990-TX-97 Etherlink with 3XP Processor> port 0xb800-0xb87f mem 0xe6800000-0xe683ffff irq 12 at device 10.0 on pci0
txp0: Ethernet address: 00:01:03:d4:91:4f