serial: 8250_exar: Describe all members in struct exar8250_board

Describe all memebers in struct exar8250_board, otherwise we get a warning:

drivers/tty/serial/8250/8250_exar.c:122: warning: Function parameter or member 'has_slave' not described in 'exar8250_board'
drivers/tty/serial/8250/8250_exar.c:122: warning: Function parameter or member 'setup' not described in 'exar8250_board'
drivers/tty/serial/8250/8250_exar.c:122: warning: Function parameter or member 'exit' not described in 'exar8250_board'

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>