spi: spi-mux: Fix coverity issue, unchecked return value

The return value of spi_setup() is not captured within
spi_mux_select() and it is assumed to be always success.

CID: 1638374

Signed-off-by: Sergio Perez Gonzalez <sperezglz@gmail.com>
Link: https://patch.msgid.link/20250316054651.13242-1-sperezglz@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>