MIPS: Alchemy: Fix an out-of-bound access in db1200_dev_setup()

When calling spi_register_board_info(), we should pass the number of
elements in 'db1200_spi_devs', not 'db1200_i2c_devs'.

Fixes: 63323ec5

 ("MIPS: Alchemy: Extended DB1200 board support.")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>