serial/amba-pl011: Fix mismerge between v4.1-rc4 and tty-next

In commit 02730d3c
(Merge 4.1-rc4 into tty-next), git mismerged some lines,
reintroducing a reference to the removed field
uart_amba_port.tx_irq_seen.  This causes a build failure.

This patch removes the mismerged lines, restoring the code to what
was in tty-next (which was the intention).

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>