drivers: flash: flexspi: Fix XIP during flash write

[mstumpf-vected]

Fixes #83231.

XIP prefetching seems to still be running, even while interrupts are disabled. Therefore it is important to wait for the FlexSPI to be idle before performing a write/erase operation.

[github-actions]

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[mstumpf-vected]

FYI, I already contributed previously, this is my work account. My private account is @Finomnis.

[danieldegrasse]

Fix looks correct to me

[Finomnis]

One more :)

[github-actions]

Hi @mstumpf-vected!
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