spi/bcm63xx: reject transfers unable to transfer

The hardware does not support keeping CS asserted after sending one
FIFO buffer worth of data, so reject transfers requiring CS being kept
asserted, either between transers or for a certain time after it,
or exceeding the FIFO size.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>