usb_modeswitch: set CXX to false when C++ is missing

usb_modeswitch: set CXX to false when C++ is missing

Similar to the openocd fix in commit 5966e2dc54 (package/openocd: fix
fallout after no-C++ fixups) the jimctl that is bundled with
usb_modeswitch also wants to find a binary. This broke with commit
4cd1ab158 (core: alternate solution to disable C++). Revert to 'false'
instead of 'no' here as well.

Fixes:
http://autobuild.buildroot.net/results/b4d/b4d4e925763cb6558af915f9781afe07fc557ebc/
http://autobuild.buildroot.net/results/61b/61b9dc2148df2e8fd0b30e62aedbfd30bb755e19/
http://autobuild.buildroot.net/results/468/468a7e6b049f159fbb4e79d3a12b53ca890f6933/

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>