spi: stm32_qspi: Remove useless struct stm32_qspi_flash

spi: stm32_qspi: Remove useless struct stm32_qspi_flash

Currently, in stm32_qspi_claim_bus(), QSPI_CR and QSPI_DCR registers
are saved in stm32_ospi_flash struct on first flash memory initialization
and restored on each flash accesses.

As the logic of spi-uclass.c changed since 'commit 0bd78b3ec80d
("spi: spi-uclass: Fix spi_claim_bus() speed/mode setup logic")'
set_speed() and set_mode() callbacks are called systematically when bus
speed or bus mode need to be updated, QSPI_CR and QSPI_DCR registers are
set accordingly.

So stm32_qspi_claim_bus() can be updated by removing QSPI_CR and QSPI_DCR
save/restore code and struct stm32_ospi_flash can be removed as well.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>