m68k/mac: Adopt naming and calling conventions for PRAM routines

Adopt the existing *_read_byte and *_write_byte naming convention.
Rename via_pram_readbyte and via_pram_writebyte to avoid confusion.
Adjust calling conventions of mac_pram_* functions to match the
struct nvram_ops methods.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>