drivers: nfc: nfcsim: remove return value check of `dev_dir`

Smatch complains that:
nfcsim_debugfs_init_dev() warn: 'dev_dir' is an error pointer or valid

According to the documentation of the debugfs_create_dir() function,
there is no need to check the return value of this function.
Just delete the dead code.

Signed-off-by: Jianuo Kuang <u202110722@hust.edu.cn>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230424024140.34607-1-u202110722@hust.edu.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>