spi: spi-stm32-qspi: Remove CR_FTHRES_MASK usage

On STM32 F4/F7/H7 SoCs, FTHRES is a 5 bits field in QSPI_CR register,
but for STM32MP1 SoCs, FTHRES is a 4 bits field long. CR_FTHRES_MASK
definition is not correct.

As for all these SoCs, FTHRES field is set to 3, FIELD_PREP() macro
is used with a constant as second parameter which make its usage useless.

CR_FTHRES_MASK and FIELD_PREP() can be removed.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>