serial: sirf: make fifo functions static

The fifo mask functions should be static as they are not
used outside of the driver. Fix the following warnings by
making them static:

drivers/tty/serial/sirfsoc_uart.h:109:5: warning: symbol 'uart_usp_ff_full_mask' was not declared. Should it be static?
drivers/tty/serial/sirfsoc_uart.h:117:5: warning: symbol 'uart_usp_ff_empty_mask' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sirfsoc_uart.h

@@ -106,15 +106,15 @@ struct sirfsoc_uart_register {
 	enum sirfsoc_uart_type uart_type;
 };
 
-u32 uart_usp_ff_full_mask(struct uart_port *port)
+static u32 uart_usp_ff_full_mask(struct uart_port *port)
 {
 	u32 full_bit;
 
 	full_bit = ilog2(port->fifosize);
 	return (1 << full_bit);
 }
 
-u32 uart_usp_ff_empty_mask(struct uart_port *port)
+static u32 uart_usp_ff_empty_mask(struct uart_port *port)
 {
 	u32 empty_bit;