spi: fsl-qspi: Reprogram the clock rate when the operation frequency changes

fsl_qspi_select_mem() returns early when the chip select has not changed,
which happens before it reaches clk_set_rate(). Since the rate is now
taken from the spi-mem operation rather than from the SPI device, the
controller honours op->max_freq exactly once per chip select and ignores
it for every operation after that.

q->selected is only reset to -1 in fsl_qspi_default_setup(), i.e. at probe
and on resume, so on the common single chip select board the very first
operation latches a rate that all subsequent operations inherit, whatever
frequency they asked for.

This results in operations being issued with the wrong frequency.

Cache the operation frequency the clock was programmed for next to the
selected chip select, and redo the clock setup when either changes.

Fixes: 2438db5253eb ("spi: fsl-qspi: Support per spi-mem operation frequency switches")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Acked-by: Han Xu <han.xu@nxp.com>
Link: https://patch.msgid.link/20260917-fsl-qspi-freq-op-fix-v1-1-5fbe6b02f738@kontron.de
Signed-off-by: Mark Brown <broonie@kernel.org>