contrib/udev: explicitly load the sg module in the udev rule

We never got around to figuring out why the `sg` module is auto-loaded on
some systems/distributions (e.g. Debian) but not on others (e.g. Arch).
We have since used workarounds like adding `sg` to `/etc/modules`.

Instead just use udev to load the module if a new USB-SD-Mux is connected,
as suggested by @frosteyes in [1].

[1]: #87

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>

contrib/udev/99-usbsdmux.rules

@@ -1,3 +1,5 @@
 # USB-SD-Mux
 ACTION=="add", SUBSYSTEM=="scsi_generic", KERNEL=="sg[0-9]*", ATTRS{manufacturer}=="Pengutronix", ATTRS{product}=="usb-sd-mux*", SYMLINK="usb-sd-mux/id-$attr{serial}", TAG+="uaccess", GROUP="plugdev"
 ACTION=="add", SUBSYSTEM=="scsi_generic", KERNEL=="sg[0-9]*", ATTRS{manufacturer}=="Linux Automation GmbH", ATTRS{product}=="usb-sd-mux*", SYMLINK="usb-sd-mux/id-$attr{serial}", TAG+="uaccess", GROUP="plugdev"
+ACTION=="add", SUBSYSTEM=="usb", ATTRS{manufacturer}=="Pengutronix", ATTRS{product}=="usb-sd-mux*", RUN{builtin}+="kmod load sg"
+ACTION=="add", SUBSYSTEM=="usb", ATTRS{manufacturer}=="Linux Automation GmbH", ATTRS{product}=="usb-sd-mux*", RUN{builtin}+="kmod load sg"