drivers: flash: stm32_qspi: Fix flash configuration in dual flash mode

When the QSPI is used in dual flash mode, e.g. on the STM32H747I-DISCO
board, only the first flash memory was reset and configured in 4-byte
addressing mode. This was in particular causing data to be incorrectly
read from the second flash memory and only even bytes were valid during
a read.

The dual flash mode was only enabled after reading the SFDP table since
it is desired to read the table of only one flash memory, not both.

This commit changes the driver to only disable the dual mode temporarily
while reading the SFDP table, ensuring all other commands and in
particular configuration commands are sent to both flash memories.

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