The procedure to build only a specific portion of the kernel is as follows:
From the root directory ( /usr/src/linux or what have you ) issue make, with the desired subdirectory as a target. For example:
sudo make drivers/usb/storage
would accomplish what you describe.
Note, though, that what you are requesting is somewhat unusual. Ordinarily, one would build the entire source tree. They might then edit some kernel code. At that point, you can just rebuild the kernel and GNU make will only recompile those areas of the tree that you affected -- in effect, doing so quite quickly.