spi: bcm-qspi: Drive MSPI peripheral SSb pin on cs_change

As per the spi core implementation for MSPI devices when the transfer is
the last one in the message, the chip may stay selected until the next
transfer. On multi-device SPI busses with nothing blocking messages going
to other devices, this is just a performance hint; starting a message to
another device deselects this one. But in other cases, this can be used
to ensure correctness. Some devices need protocol transactions to be built
from a series of spi_message submissions, where the content of one message
is determined by the results of previous messages and where the whole
transaction ends when the chipselect goes intactive.

On CS change after completing the last serial transfer, the MSPI driver
drives SSb pin CDRAM register correctly according comments in core spi.h
as shown below:

case 1) EOM =1, cs_change =0: SSb inactive
case 2) EOM =1, cs_change =1: SSb active
case 3) EOM =0, cs_change =0: SSb active
case 4) EOM =0, cs_change =1: SSb inactive

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Link: https://lore.kernel.org/r/20200420190853.45614-5-kdasu.kdev@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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