spi: dw: Allow bits_per_word to be configured by device drivers

The DesignWare SPI controller supports configurable bits_per_word
(typically 4-32 bits), but this was previously hardcoded to 8 bits
in the driver initialization.

This patch enables bits_per_word to be set dynamically by upper-level
device drivers, matching the approach used in Linux. The controller
reads the bits_per_word value from the spi_slave structure during
each transfer, allowing different SPI devices on the same bus to use
different word sizes.

Implementation details:
 - Read slave->bits_per_word in dw_spi_xfer() before each transfer
 - Validate requested value against controller capabilities (4 to max_xfer)
 - Default to 8 bits if not set (maintains backward compatibility)

This follows the Linux model where spi_device drivers set bits_per_word,
and the controller driver reads it in the transfer function. Device
drivers can now set slave->bits_per_word before calling spi_xfer().

Example usage in device driver:
 slave->bits_per_word = 16;
 spi_xfer(slave, ...);

Backward compatible: Existing drivers that don't set bits_per_word
will continue to work with the default 8-bit transfers.

Signed-off-by: Boon Khai Ng <[email protected]>

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