spi: microchip-core: defer asserting chip select until just before write to TX FIFO

Setting up many of the registers for a new SPI transfer requires the
SPI controller to be disabled after set_cs() has been called to assert
the chip select line. However, disabling the controller results in the
SCLK and MOSI output pins being tristate, which can cause clock
transitions to be seen by a slave device whilst SS is active. To fix
this, the CS is only set to inactive inline, whilst setting it active
is deferred until all registers are set up and the any controller
disables have been completed.

Fixes: 9ac8d176

 ("spi: add support for microchip fpga spi controllers")
Signed-off-by: Steve Wilkins <steve.wilkins@raymarine.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240715-sanitizer-recant-dd96b7a97048@wendy
Signed-off-by: Mark Brown <broonie@kernel.org>