net: txgbe: initialize num_q_vectors for MSI/INTx interrupts

When using MSI/INTx interrupts, wx->num_q_vectors is uninitialized.
Thus there will be kernel panic in wx_alloc_q_vectors() to allocate
queue vectors.

Fixes: 3f703186

 ("net: libwx: Add irq flow functions")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>