Atmel AIC has common structure for SMR (Source Mode Register).
bit[6:5] Interrupt source type
bit[2:0] Priority level
Other bits are unused.
To update new priority value, bit[2:0] should be cleared first and then
new priority level can be written. However, aic_common_set_priority()
helper clears source type bits instead of priority bits.
This patch fixes wrong mask bit operation.