spi: stm32h7: replace private SPI_1HZ_NS with NSEC_PER_SEC

Replace SPI_1HZ_NS private constant with NSEC_PER_SEC, which is easier
to read and understand.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/1612551572-495-8-git-send-email-alain.volmat@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>