spi: fsi: Batch TX operations

Batch sequential write transfers up to the max TX size (40 bytes).
This controller must specify a max transfer size of only 8 bytes for
RX operations.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://patch.msgid.link/20250131200158.732898-1-eajames@linux.ibm.com


Signed-off-by: Mark Brown <broonie@kernel.org>