Fix low-level uart drivers that set their fifo sizes in the softc too late.

uart(4) allocates send and receiver buffers in attach() before it calls
the low-level driver's attach routine.  Many low-level drivers set the
fifo sizes in their attach routine, which is too late.  Other drivers set
them in the probe() routine, so that they're available when uart(4)
allocates buffers.  This fixes the ones that were setting the values too
late by moving the code to probe().