drivers: flash: xspi: Put delay block config under specific bit

On some series, use of Delay Block could be possible on OSPI device but
not on HSPI one (which uses this drivers).
As a quick fix check the presence of XSPI Delay Block by-pass
configuration bit instead of the Delay Block presence.

Note: This fix works because we don't have cases today where this driver
is used for OSPI and HSPI instances with mixed DLYB compatibility.
This may have to be reviewed one day and may require a more complex fix
with instantiable configuration, but this day may never come as well.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>