wlcore: spi: use private max-buf-size limit

Limit SPI transmissions to the wl12xx max buffer size, as only 12xx was
tested with SPI.

This allows us to remove the global aggregation buffer constant.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>