drivers: flash: stm32 ospi: Limit bytes read from DT SFDP table

In case SFDP table is provided via device tree, take care not reading
more than expected by the function caller as the result is written
in a structure which size is predefined by one specific byte in the
table, and could be smaller than the table size.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>