Couple minor if_vmx tweaks

• Use queue size fields from the Tx/Rx queues in various places instead of (currently the same values) from the softc.
• Fix potential crash in detach if the attached failed to alloc queue memory.
• Move the VMXNET3_MAX_RX_SEGS define to a better spot.
• Tweak frame size calculation w.r.t. ETHER_ALIGN. This could be tweaked some more, or removed since it probably doesn't matter much for x86 (and the x86 class of machines this driver will be used on).