stm32l4x.c: Correct waiting for data.

Old code waited only for 7 bytes and didn't handle buffer wrap-around, but
was functional despite.

Change-Id: Iceaf7be1e51368b2ec0a8722cc9ac16d12f9aa63
Signed-off-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-on: http://openocd.zylin.com/3140


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>