Fix CS8900 regression on impa7 board

The following error was seen on impa7 board, due to its use of a 32-bit bus
on CS8900.
cs8900.c:137:37: error: macro "get_reg_init_bus" passed 2 arguments, but takes just 1

This patch gives the macro the correct number of arguments

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>