net: stmmac: use more descriptive names in stmmac_xmit()

Use "frag_size" rather than "len", correcting its type to be
unsigned int. Rename "des" to "dma_addr" since that's what it is.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w2vbV-0000000DbWi-1O80@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>