SPI: mxc_spi: delay initialisation until claim bus

it is not correct to init for a specific slave in spi_setup_slave.
instead buffer the values and delay init until spi_claim_bus.

Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>