usb: Fix BCD() macro used to set bcdDevice

According to API documentation, the bcdDevice field of the USB
descriptor is supposed to represents the Zephyr kernel major
and minor versions as a binary coded decimal value. However,
when using zephyr 2.0, bcdDevice is shown as 0.00 instead of 2.00.

This is due to a typo in the implementation of the BCD macro in
usb_commond.h. This commit fixes the macro.

Signed-off-by: Arnaud Taffanel <arnaud@bitcraze.io>