spi: tegra114: Preserve SPI mode bits in def_command1_reg

spi: tegra114: Preserve SPI mode bits in def_command1_reg

The COMMAND1 register bits [29:28] set the SPI mode, which controls
the clock idle level. When a transfer ends, tegra_spi_transfer_end()
writes def_command1_reg back to restore the default state, but this
register value currently lacks the mode bits. This results in the
clock always being configured as idle low, breaking devices that
need it high.

Fix this by storing the mode bits in def_command1_reg during setup,
to prevent this field from always being cleared.

Fixes: f333a331adfa ("spi/tegra114: add spi driver")
Signed-off-by: Vishwaroop A <va@nvidia.com>
Link: https://patch.msgid.link/20260204141212.1540382-1-va@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>