spi: atmel-quadspi: Avoid overwriting delay register settings

spi: atmel-quadspi: Avoid overwriting delay register settings

Previously the MR and SCR registers were just set with the supposedly
required values, from cached register values (cached reg content
initialized to zero).

All parts fixed here did not consider the current register (cache)
content, which would make future support of cs_setup, cs_hold, and
cs_inactive impossible.

Setting SCBR in atmel_qspi_setup() erases a possible DLYBS setting from
atmel_qspi_set_cs_timing().  The DLYBS setting is applied by ORing over
the current setting, without resetting the bits first.  All writes to MR
did not consider possible settings of DLYCS and DLYBCT.

Signed-off-by: Alexander Dahl <ada@thorsis.com>
Fixes: f732646d0ccd ("spi: atmel-quadspi: Add support for configuring CS timing")
Link: https://patch.msgid.link/20240918082744.379610-2-ada@thorsis.com
Signed-off-by: Mark Brown <broonie@kernel.org>