sh: pfc: Convert index to field and value pair

Update the way the PFC code is passing bitfield
selection between configure register functions.

Convert the code from using index only to bitfield
number and selected value. First step towards future
variable bitfield width support.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>