spi: stm32-ospi: Increase read throughput in indirect mode

spi: stm32-ospi: Increase read throughput in indirect mode

Schedule() is called every u8/u16 or u32 read accesses which is overkill.
Move schedule() from stm32_ospi_read_fifo() to stm32_ospi_tx_poll()
and call schedule() only every 1MB chunk of data.

Test performed by reading 64MB on sNOR on stm32mp257f-ev1 board:

          before      after    ratio
Read  :  10.6MB/s    14.2MB/s   +34%

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