spi/bcm63xx: work around inability to keep CS up

This SPI controller does not support keeping CS asserted after sending
a transfer.
Since messages expected on this SPI controller are rather short, we can
work around it for normal use cases by sending all transfers at once in
a big full duplex stream.

This means that we cannot change the speed between transfers if they
require CS to be kept asserted, but these would have been rejected
before anyway because of the inability of keeping CS asserted.

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