Small fix for adc_isbusy()
Small fix for adc_isbusy()

Do not read ADC status register twice. AVR32_ADC.SR.drdy rereads the
status, so instead of doing that use AVR32_ADC_SR_DRDY_MASK with the __adc_lsr.

aery32/aery32,denravonska/aery32,aery32/aery32,denravonska/aery32