mfd: dln2: Fix _dln2_transfer return code

If wait_for_completion_interruptible_timeout returns a positive value
it may be propagated as the return value of _dln2_transfer. This
contradicts the documentation of the function and exposes unnecessary
internals to the callers.

This patch makes sure to set the return value to 0 in that case.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>