spi: stm32-qspi: remove signal sensitive on completion

On umount step a sigkill signal is set (without user specific
action), due to sigkill signal the completion will be interrupted and
the data transfer can't be finished if a sync is needed.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>