staging: mt7621-pinctrl: replace ENOSYS with better fitting error code

This commit replaces ENOSYS return with ENOTSUPP silencing the
following checkpatch warning:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>