drivers: spi: stm32: fix h7 issue with spi_hold_on_cs

STM32H7 spi_loopback test fails since the introduction of a test enabling
SPI_HOLD_ON_CS.
This uncovered an issue where the SPI ISR is constantly called if the SPI
is not disabled, even if the interrupt enable register is completely
cleared.
A workaround is to disable the SPI IRQ at the NVIC level when
SPI_HOLD_ON_CS is used.


Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>