Need to set SPI clock speed

[longears]

Right now we're not setting the SPI clock speed.

Here is demo code doing it in C:
https://github.com/zestyping/openpixelcontrol/blob/master/src/spi.c#L58

We need to reproduce this in Go.

Because we don't have this, controlling pixels from a Beaglebone will result in noisy flickering.

A temporary workaround is to run the linked C code once at boot, which will set the SPI clock speed until the next boot.

[fragmede]

It's possible according to the syscall package docs: http://golang.org/pkg/syscall/
Something like
x,y,z = syscall.Syscall(SYS_IOCTL, SPI_IOC_WR_MAX_SPEED_HZ, uinptr(speed));
might do the trick.