input: atmel_mxt_ts: Fix section mismatch

WARNING: vmlinux.o(.devinit.text+0x4878): Section mismatch in
reference from the function mxt_probe() to the function
.init.text:mxt_debugfs_init()
The function __devinit mxt_probe() references
a function __init mxt_debugfs_init().
If mxt_debugfs_init is only used by mxt_probe then
annotate mxt_debugfs_init with a matching annotation.

Change-Id: Iebb2b174e307d2497297b16396579c7ef3da6721
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>