kernel: fix fmc find device

The usage of on-stack-variable by the callback function was a problem
on some kernels (e.g. 3.6 64bit)

Federico: again I'm including this patch because we have not time to
wait the proper solution that will come on october 2014 (hopefully)

Signed-off-by: Tomasz Wlostowski <tomasz.wlostowski@cern.ch>
Acked-off-by: Federico Vaga <federico.vaga@cern.ch>