Fix a bug brought to light by the people working on SMPng. I don't quite

understand exactly what it is about SMPng that tickles this bug. What I
do know is that the foo_init() routine in most drivers is often called
twice when an interface is brought up. One time is due to the ifconfig(8)
command calling the SIOCSIFFLAGS ioctl to set the IFF_UP flag, and another
is probably due to the kernel calling ifp->if_init at some point. In any
case, the SMPng changes seem to affect the timing of these two events in
such a way that there is a significant delay before any packets are sent
onto the wire after the interface is first brought up. This manifested
itself locally as an SMPng test machine which failed to obtain an address
via DHCP when booting up.

It looks like the second call to fxp_init() is happening faster now than
it did before, and I think it catches the chip while it's in the process
of dealing with the configuration command from the first call. Whatever
the case, a FXP_CSR_SCB_CNA interrupt event is now generated shortly after
the second fxp_init() call. (This interrupt is apparently never generated
by a non-SMPng kernel, so nobody noticed.)

There are two problems with this: first, fxp_intr() does not handle the
FXP_CSR_SCB_CNA interrupt event (it never tests for it or does anything
to deal with it), and second, the meaning of FXP_CSR_SCB_CNA is not
documented in the driver. (Apparently it means "command unit not active.")
Bad coder. No biscuit.

The fix is to have the FXP_CSR_SCB_CNA interrupt handled just like the
FXP_SCB_STATACK_CXTNO interrupt. This prevents the state machine for
the configuration/RX filter programming stuff from getting wedged for
several seconds and preventing packet transmission.

Noticed by: jhb

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