drivers: usb: stm32: Simplify Kconfig help message

The USB_DC_STM32 help message started to miss some
STM32 MCU families. Overtime, the message will
get bigger if we continue to list family names.
Removed family names to simplify the message and
avoid periodic modifications.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>