spi: rockchip: avoid objtool warning

spi: rockchip: avoid objtool warning

Building this file with clang leads to a an unreachable code path
causing a warning from objtool:

drivers/spi/spi-rockchip.o: warning: objtool: rockchip_spi_transfer_one()+0x2e0: sibling call from callable instruction with modified stack frame

Change the unreachable() into an error return that can be
handled if it ever happens, rather than silently crashing
the kernel.

Fixes: 65498c6ae241 ("spi: rockchip: support 4bit words")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210226140109.3477093-1-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>