spi: stm32_qspi: avoid warnings when building with W=1 option

This patch solves warnings detected by setting W=1 when building.

Warnings type detected:
 - [-Wtype-limits]
 - [-Wsign-compare]

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>