i2c: dln2: Simplify return flow for dln2_i2c_enable

This fixes the following kbuild test robot warning:

>> drivers/i2c/busses/i2c-dln2.c:70:1-4: WARNING: end returns can be simplified if negative or 0 value

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>