spi: stm32-qspi: Fix compilation warning in ARM64

This fixes warnings detected when compiling in ARM64.
Introduced by 'commit 18674dee

 ("spi: stm32-qspi: Add dirmap support")'

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/20210420082103.1693-1-patrice.chotard@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>