spi: stm32: qspi: Clamp stm32_qspi_get_mode() output to CCR_BUSWIDTH_4

In case usage of OCTAL mode, buswidth parameter can take the value 8.
As return value of stm32_qspi_get_mode() is used to configure fields
of CCR registers that are 2 bits only (fields IMODE, ADMODE, ADSIZE,
 DMODE), clamp return value of stm32_qspi_get_mode() to 4.

Fixes: a557fca6

 ("spi: stm32_qspi: Add transfer_one_message() spi callback")
Cc: stable@vger.kernel.org
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://msgid.link/r/20240618132951.2743935-3-patrice.chotard@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>