sh-pfc: Fix compiler warning when BUG()

The sh_pfc_phys_to_virt() function ends with a BUG() statement without a
return. When CONFIG_BUG isn't set the function will thus have no return
value. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>