nokia skip-reset: make consistent with other vty settings
All other Nokia-specific vty settings use is_nokia_bts() helper function, while the code for skip-reset directly compares bts->type against GSM_BTS_TYPE_NOKIA_SITE. Change to use the helper function: it is consistent with other vty settings, and avoids one more place that erroneously refers to *Site instead of all Nokia BTS.
