package/usb_modeswitch: disable parallel build

Build of package will sometime fails because of the following issue:
install-static target has two dependencies: dispatcher-static and
install-common

Because dispatcher-static is not a file but only a target, it will
always be called to build usb_modeswitch_dispatcher.
So, even if install-common depends on usb_modeswitch_dispatcher, in some
rare cases, install-static won't be able to install
usb_modeswitch_dispatcher because it is being rebuild by
dispatcher-static

To fix this issue, disable parallel build

Fixes:
 - http://autobuild.buildroot.org/results/8297be35725b816ff5afaf909605ceb41223efb6



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>