commit 1ff383a4 upstream.

This patch adds waiting until transmit buffer and shifter will be empty
before clock disabling.

Without this fix it's possible to have clock disabled while data was
not transmited yet, which causes unproper state of TX line and problems
in following data transfers.
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>