boards: arm: thingy53: Fix default USB related log levels

Previous method of setting default USB related log levels for Thingy53
led to generating inconsistent configuration, ex.
CONFIG_USB_CDC_ACM_LOG_LEVEL=0 and CONFIG_USB_CDC_ACM_LOG_LEVEL_INF=y
at the same time. This commit changes to proper implementation
which is overlaying choice's default.

Signed-off-by: Mateusz Kapala <mateusz.kapala@nordicsemi.no>