serial: pl011: Fix build breakage with !CONFIG_DMA_ENGINE

Commit 479e9b94fdce7bc4 ("serial: Refactor uart_flush_buffer() from
uart_close") refactored the uart_flush_buffer() in uart_close() into
those drivers that define a flush_buffer() method. However, in the
amba-pl011 driver configured without CONFIG_DMA_ENGINE, flush_buffer()
is a NULL method, so the direct call fails to compile.

Check and call the flush_buffer() method through the ops table instead.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 36f339d136f3b6e91235be619a88289a5f980450
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git



CRs-Fixed: 2037829
Change-Id: I54ae7b2aa4c9274f745d39747442ac61f900c52f
Signed-off-by: Atul Raut <araut@codeaurora.org>