Help, mouse and touchpad scrolls in opposite directions

[Johannes Lundberg]

Hi X11 wizards

I have a Dell Latitude E7270 laptop where I sometimes use the built in
touchpad and sometimes an external mouse. but the touchpad and the mouse
scrolls in opposite directions for the same setting.
For mouse I have to run
xmodmap -e "pointer = 1 2 3 4 5"
and for touchpad
xmodmap -e "pointer = 1 2 3 5 4"

I'm not using any xorg.conf, everything is default, except the xmodmap
command in .xinitrc. The only related kernel modules loaded are ums and
uhid.

Can I configure the system somehow so that I don't manually have to change
the modmap every time I switch between mouse/touchpad?