spi: stm32: Simplify stm32h7_spi_prepare_fthlv()

Simplify stm32h7_spi_prepare_fthlv() function implementation,
no functional change intended.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alain Volmat <alain.volmat@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Cc: Antonio Borneo <antonio.borneo@st.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Roman Guskov <rguskov@dh-electronics.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-spi@vger.kernel.org
Link: https://lore.kernel.org/r/20210104123114.261596-1-marex@denx.de


Signed-off-by: Mark Brown <broonie@kernel.org>