On SVEC we can have up to two FMC-ADC-100M mezzanine, potentially
triggering at the same time. If such conditions, the driver fails in
performing the DMA on both mezzanine because one of the two will fail to
request a channel.

With this patch we introducte a 10s timeout for the ADC to request a DMA
channel on SVEC.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>