I know that this is an old thread, but there's no sense in leaving a question unanswered
You've got two options: set it manually by editing `.config` and setting it to =y, or use menuconfig:
Option 1
Code:
vim /usr/src/linux/.config
Option 2
Code:
cd /usr/src/linux/
make menuconfig
Navigate to Device Drivers > USB Support > USB Serial Support > Generic USB Serial support
Enable it.