mtkmodem: naming refactors

The original naming in _plugins/mtk.c_ is pretty confusing. __GRil__ instances are generally named _ril_ in other files, but that's also used for __struct mtk_data__ here. And the __GRil__ typed member inside __struct mtk_data__ is named _modem_, which conflicts with the __ofono_modem__ typed member again. This patch ensures following naming rule is applied.

 * struct mtk_data => mtk
 * GRil => ril
 * ofono_modem => modem
md_[0|1] seems too short to me for a global variable, and the name does not really convey information. Please keep it as mtk_[0|1] or even better, change them to mtk_data_[0|1].