For what controller?
There are some .c/.h files related to controllers inside "kernel/", but they are split according to what controller they are.
PS3Controller.c/.h and BT.c/.h are the ones for PS3 and Bluetooth controllers (BT being Wiimote, CC, CCP, etc).
PADReadGC.c is the one which seems to contains most of the button mapping and Analog sticks code for all the controllers, perhaps you could find something there too.
Thanks! I am mainly interested in adjusting analog sticks for Gamecube controllers (both as native for Wii, and HID for Wii U) and the Wii U Gamepad. I looked around but I don't see any clear way to modify sensitivity. The most relevant part seems to be lines 564-577
https://github.com/FIX94/Nintendont/blob/master/loader/source/ppc/PADReadGC.c