net: dwc_eth_qos: Use lower_32_bits() for tail pointers

net: dwc_eth_qos: Use lower_32_bits() for tail pointers

The DesignWare Cores Ethernet Quality-of-Service databook state that the
descriptor address from the start to the end of the ring must not cross
the 4GB boundary.

Use lower_32_bits() to write the lower 32 bits of descriptor addresses,
including the 32-bit tail pointers, consistently. No functional change
is intended.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>