When the DMA table has been allocated dynamically, the IOCTL_SET_PACKETSIZE
will trigger an issue where pDev->rx and pDev->tx are not updated with
the new DMA tables base address. Instead the old pointers are used.
There is no point in reallocting the DMA tables because there is no
configuration option to it. Therefore the DMA tables allocation is
moved to a separate function never called from SET_PACKETSIZE.
