net: libwx: fix the using of Rx buffer DMA

In the Linux kernel, the following vulnerability has been resolved:

net: libwx: fix the using of Rx buffer DMA

The wx_rx_buffer structure contained two DMA address fields: 'dma' and

'page_dma'. However, only 'page_dma' was actually initialized and used

to program the Rx descriptor. But 'dma' was uninitialized and used in

some paths.

This could lead to undefined behavior, including DMA errors or

use-after-free, if the uninitialized 'dma' was used. Althrough such

error has not yet occurred, it is worth fixing in the code.