Kernel - Completely remove DFLTPHYS

Kernel - Completely remove DFLTPHYS

* There is no need to have two different macros define the maximum
  allowed IO size for low-level operations; replace DFLTPHYS by
  MAXPHYS in the entire kernel.

* Old drivers typically used DFLTPHYS without a second thought,
  assuming it would always be the same value; they may have hard
  limitations and break if bigger I/O sizes are suddenly used.
  Be sure to explicitely limit their maximum allowed I/O size to
  64KB and keep them working.