spi: Fix the Pcs setup for NXP's K6xF processors

Without this change, when DTS SPI device node has node ID = <0>, the
value of 0 is assigned during SPI configuration and written to
whichPcs member in master_config structure.
This value wrongly overrides the default value read from NXP's DSPI
HAL (kDSPI_Pcs0 = 1U << 0).

Such situation occurs on ip_k66f board, where the DSA device -
controlled via SPI has the node ID equal to 0 (i.e. reg = <0>).

Signed-off-by: Lukasz Majewski <lukma@denx.de>