Remove struct stm32_qspi_flash's field qspi which is not used.

Fixes: c530cd1d ("spi: spi-mem: add stm32 qspi controller")
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/20210615090115.30702-1-patrice.chotard@foss.st.comSigned-off-by: Mark Brown <broonie@kernel.org>