serial: sirf: fix the amount of serial ports

SiRFprimaII has three uart ports and three USP-based ports, so there
are totally six lines instead of five.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>