spi: meson-spicc: adapt burst handling for G12A support

The G12A SPICC controller variant has a different FIFO size and doesn't
handle the RX Half interrupt the same way as GXL & AXG variants.

Thus simplify the burst management and take in account a variable FIFO
size.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20200312133131.26430-8-narmstrong@baylibre.com


Signed-off-by: Mark Brown <broonie@kernel.org>