i2c: viperboard: remove superfluous assignment

cppcheck rightfully says:

drivers/i2c/busses/i2c-viperboard.c:169: style: Variable 'bytes_xfer' is assigned a value that is never used.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Lars Poeschel <poeschel@lemonage.de>