usb: gadget: u_serial: Use kfifo instead of homemade circular buffer

The kernel FIFO implementation, kfifo, provides interfaces to manipulate
a first-in-first-out circular buffer.  Use kfifo instead of the homemade
one to make the code more concise and readable.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>