drivers: adc: configurable wait for completion timeout

Depending on the ADC implementation it might happen that
the driver is waiting on an external interrupt. If this interrupt
gets lost, for instance due to a race condition with an external
port expander, the system will get stuck.
Making this configurable allows the user to recover from such
an error.

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>