try this instead. rather than marking the register "not enabled" consider turning off the clock to the peripheral in question
PM->APBBMASK.reg &= ~PM_APBBMASK_PORT;
PM->APBBMASK.reg &= ~PM_APBBMASK_DMAC;
PM->APBCMASK.reg &= ~PM_APBCMASK_SERCOM0;
PM->APBCMASK.reg &= ~PM_APBCMASK_SERCOM1;
and so on.
here's the power manager's bus bitmasks:
AHBMASK: CLK_HPBA_AHB CLK_HPBB_AHB CLK_HPBC_AHB CLK_DSU_AHB CLK_NVMCTRL_AHB CLK_DMAC_AHB CLK_USB_AHB
APBAMASK: CLK_PAC0_APB CLK_PM_APB CLK_SYSCTRL_APB CLK_GCLK_APB CLK_WDT_APB CLK_RTC_APB CLK_EIC_APB
APBBMASK: CLK_PAC1_APB CLK_DSU_APB CLK_NVMCTRL_APB CLK_PORT_APB CLK_DMAC_APB CLK_USB_APB
APBCMASK: CLK_SERCOM0_APB CLK_SERCOM1_APB CLK_SERCOM2_APB CLK_SERCOM3_APB CLK_SERCOM4_APB CLK_SERCOM5_APB CLK_TCC0_APB CLK_TCC1_APB CLK_TCC2_APB CLK_TC3_APB CLK_TC4_APB CLK_TC5_APB CLK_ADC_APB CLK_DAC_APB