[embassy-stm32] OSPI/HSPI/XSPI transfer fixes

[bschwind]

This PR fixes a few issues:

• Using a literal 0 instead of a more informative enum variant for describing the "indirect write" mode
• Transfers over OSPI, HSPI, and XSPI didn't take into account the word size when setting the register for the number of bytes to transfer
• One of our previous PRs (embasy-stm32: Allow large OSPI DMA writes with chunking #4671), also didn't take into account the word size when chunking DMA transfers. Sorry about that! Thankfully we caught this before the next embassy release, as before embasy-stm32: Allow large OSPI DMA writes with chunking #4671, it would have panicked on a transfer size larger than 0xFFFF anyway

I would normally pull 0xFFFF out into something like const MAX_DMA_TRANSFER_BYTES: usize = 0xFFFF and place that in the GPDMA module for embassy-stm32, but I'm not sure if that value holds true for all STM32 variants. If that is true, then I think the code clarity would benefit from having it defined in a const.