staging: comedi: ni_660x: remove enum ni_register_width

All the registers are defined struct NI_660xRegisterData and they are
either 2 or 4 bytes in size. Remove the enum and just use a char member
to define the size as 2 or 4 bytes.

Simplify the ni_660x_{write,read}_register() functions and remove the
unnecessary BUG() in each.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>