drivers: adc: Fix handling of invalid sampling requests

Commit aad21ecb introduced an incorrect
pattern of handling ADC sampling requests with invalid parameters in
both nRF ADC drivers. After discarding such request, the drivers do not
release properly the access lock and therefore become unusable.
Unfortunately, this pattern were later on copied in all other ADC
drivers in the source tree.
This commit adds the proper lock releasing in all the affected drivers.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>