staging: comedi: s626: remove duplicate macros I2C_B0, I2C_B1, I2C_B2

The `I2C_B0(ATTR, VAL)`, `I2C_B1(ATTR, VAL)` and `I2C_B2(ATTR, VAL)`
macros are defined identically in "s626.h" and "s626.c".  Remove the
duplicates from "s626.c".

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>