iio:adc:berlin2-adc: enable interrupts with mutex locked

Move the call to enable channel interrupts into its _read() function to
have it protected by a mutex. This ensures that only one channel is
sampled at a time.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>