net: stmmac: remove tx_tail_addr

net: stmmac: remove tx_tail_addr

There is only one place where tx_q->tx_tail_addr is used - the new
stmmac_set_queue_tx_tail_ptr(). Make this a local variable and remove
it from struct stmmac_tx_queue.

This commit does not change the semantics - the hardware relies upon
the descriptor ring not crossing a 4GiB boundary as the high address
bits are programmed into a separate register via stmmac_init_tx_chan().
Hence, truncating the DMA address to 32-bit is fine as the register it
will be programmed into is 32-bit, and the high bits are handled
elsewhere.

Also change the type of desc_size to size_t, as this variable is
initialised from sizeof().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w1LWt-0000000DGT2-1oeO@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>