gpio: sim: fix uninitialized ret variable

Building with clang returns this error:

gpio-sim.c:889:7: error: variable 'ret' is uninitialized
  when used here

ret should be the status of the call to
gpio_sim_make_bank_swnode stored in bank->swnode.

Fixes: 83960fcf4818 ("gpio: sim: new testing module")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>