spi/mips-lantiq: set SPI_MASTER_HALF_DUPLEX flag

Due to hardware limitations of the spi/flash frontend of the EBU we need to set
the SPI_MASTER_HALF_DUPLEX flag.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>