serial: Remove unnecessary amba_set_drvdata()

Driver core clears the driver data to NULL after device_release
or on probe failure, so just remove it from here.

Driver core change:
"device-core: Ensure drvdata = NULL when no driver is bound"
(sha1: 0998d063)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>