ravb: correct typo in FBP field of SFO register

The field name is FBP rather than FPB.

This field is unused and could equally be removed from the driver entirely.
But there seems no harm in leaving as documentation of the presence of the
field.

Based on work by Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>