adc_disable_channel_differential_input does not correctly set config bit

[natgaertner]

ArduinoCore-sam/system/libsam/source/adc.c

Lines 632 to 635 in c893c62

	uint32_t ul_temp;
	ul_temp = p_adc->ADC_COR;
	p_adc->ADC_COR &= 0xfffeffffu << channel;
	p_adc->ADC_COR |= ul_temp;

By ORing with the previous value, the resulting of ANDing with the mask is undone, so the function results in no action being taken (once adc_enable_channel_differential_input is run, adc_disable_channel_differential_input doesn't work).
trivially with one bit, the truth table for (A and B) or A is:

A	B	(A and B) or A
T	T	T
T	F	T
F	T	F
F	F	F

I believe the desired result could be obtained by ORing with the set value (basically running adc_enable_channel_differential_input), then XORing with the same value, but I'm not a good enough C programmer to know if this is the optimal way to do this.