uart: change to call uart flush with void return type

uart: change to call uart flush with void return type

In commit e14fc739d7de ("uart: Add nonblocking IO and assorted functions")
peripheral_uart_flush function's return type was changed void to int
And, other functions using it were changed to call it with (void) type coversion
because previous code didn't check the return of flush function.
But in close function, it was ommited so add it.

Change-Id: I0f89fc71d8c4bd76a3768f2bdc96cfd52a10c2ff
Signed-off-by: TaeminYeom <taemin.yeom@samsung.com>