net: stmmac: simplify DMA descriptor allocation/init/freeing

net: stmmac: simplify DMA descriptor allocation/init/freeing

Rather than having separate branches to handle the different types of
descriptors, use the helper functions to calculate the total size of
the DMA descriptors.

Use this to allocate or free the descriptor array, and use a local
variable to hold the address of the descriptor array, so we only need
one dma_alloc_coherent() or dma_free_coherent() call in these paths.

Also do the same for the receive ring initialisation. The transmit
ring can't be converted as there is a case where stmmac_mode_init()
is not called.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w2vbQ-0000000DbWc-0ty5@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>