STM32L4 DAC: Fix naming so that DAC1 and DAC2 always refer to channels 1 and 2

User should not be bothered by details like how many IP blocks there are. As no
current STM32L4 has second DAC block (channel 3), remove support for such
hypothetical hardware. DMA channels corrected.

Change-Id: I2cba7e55803871f1ff945538113f12cf5088f68d
Signed-off-by: Juha Niskanen <juha.niskanen@haltian.com>