package/usb_modeswitch: avoid overriding variables

Overriding variables in packages recipes is an error-prone practice.

Current behavior of installing either only as a script or only as a
binary is intended, as describe in the commit log of "d3e4db4e
usb_modeswitch: bump to version 1.2.6" from 2013.

Rewrite the code to keep the same behavior while replacing variable
override [1] by conditional assignments [2].

[1]
VAR = ...
if ...
VAR = ...

[2]
if ...
VAR = ...
else
VAR = ...

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>