firewire: ohci: Remove unused function

Remove the function ar_prev_buffer_index() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/ohci.c

@@ -718,11 +718,6 @@ static inline unsigned int ar_next_buffer_index(unsigned int index)
 	return (index + 1) % AR_BUFFERS;
 }
 
-static inline unsigned int ar_prev_buffer_index(unsigned int index)
-{
-	return (index - 1 + AR_BUFFERS) % AR_BUFFERS;
-}
-
 static inline unsigned int ar_first_buffer_index(struct ar_context *ctx)
 {
 	return ar_next_buffer_index(ctx->last_buffer_index);