drivers: flash: stm32_qspi: Simplify #ifdef for dual-flash

In multiple places, "#if DT_PROP(DT_NODELABEL(quadspi), dual_flash) &&
defined(QUADSPI_CR_DFM)" was used to guard sections specific to
the dual-flash feature. This is quite long and "#ifdef
STM32_QSPI_DOUBLE_FLASH" is now used instead.

Note the presence of QUADSPI_CR_DFM is no more checked. This is not
considered as an issue since when QUADSPI_CR_DFM is not available, the
QSPI hardware doesn't support dual-flash mode so this mode must not be
enabled in the devicetree. With that change, enabling dual-flash mode
when not available causes a compile-time error.

Signed-off-by: Thomas Altenbach <altenbach.thomas@gmail.com>