spi: spi-ti-qspi: Support per-transfer and per-slave speed_hz settings

The spi-ti-qspi driver initializes its spi clock by the
spi-max-frequency property from the controller node, and ignores
per-transfer (and per-slave) speed_hz settings.

Isolate clock settings out from ti_qspi_setup() and call it from
ti_qspi_start_transfer_one() and ti_qspi_exec_mem_op(), using
per-transfer speed_hz and per-slave max_speed_hz settings.

Also drop spi_max_frequency from struct ti_qspi and use spi_master's
max_speed_hz.

Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Link: https://lore.kernel.org/r/20220519.084604.966119051165023533.atsushi.nemoto@sord.co.jp
Signed-off-by: Mark Brown <broonie@kernel.org>