Modern platforms can install more than 4GB memory, so DMA address can
larger than 32 bits. If a platform doesn't enable IOMMU, kernel needs extra
works of swiotlb to help DMA that packets reside on memory over 4GB.

The DMA addressing capability of Realtek WiFi chips is 36 bits, so set
LSB 4 bits of high 32-bit address to register and TX/RX descriptor, which
below figure shows 3-level pointers in TX direction, and RX direction is
similar but 2-level pointers only.

  +--------+
  |        | register to head of TX BD
  +---|----+
      |       +---------+
      +-----> |  TX BD  | (in memory)
              +----|----+
                   |        +---------+
                   +------> |  TX WD  | (in memory)
                            +----|----+
                                 |        +--------+
                                 +------> |   skb  |
                                          +--------+
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/20240611021901.26394-1-pkshih@realtek.com