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

[hnez]

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 #87.

I've tested the change on may Arch Linux System that would not automatically load the sg module before, but does with the updated udev rule. I've also checked if having an explicit kmod load in an (upstream) udev rule is uncommon:

$ cd /usr/lib/udev
rg kmod
rules.d/80-drivers.rules
5:ENV{MODALIAS}=="?*", RUN{builtin}+="kmod load"
6:SUBSYSTEM=="tifm", ENV{TIFM_CARD_TYPE}=="SD", RUN{builtin}+="kmod load tifm_sd"
7:SUBSYSTEM=="tifm", ENV{TIFM_CARD_TYPE}=="MS", RUN{builtin}+="kmod load tifm_ms"
8:SUBSYSTEM=="memstick", RUN{builtin}+="kmod load ms_block mspro_block"
9:SUBSYSTEM=="i2o", RUN{builtin}+="kmod load i2o_block"
10:SUBSYSTEM=="module", KERNEL=="parport_pc", RUN{builtin}+="kmod load ppdev"
11:KERNEL=="mtd*ro", ENV{MTD_FTL}=="smartmedia", RUN{builtin}+="kmod load sm_ftl"

This looks to me like they are not that common, but they exist.

Resolves #87.

[hnez]

The failed CI job is unrelated to this PR and should be fixed by #89.