spi: mux: set ctlr->bits_per_word_mask

[ Upstream commit c8bd922d

 ]

Like other SPI controller flags, bits_per_word_mask may be used by a
peripheral driver, so it needs to reflect the capabilities of the
underlying controller.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240708-spi-mux-fix-v1-3-6c8845193128@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>