serial: sirf: make fifo functions static

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>