staging: r8188eu: Fix smatch warning in rtl8188eu/core/rtw_xmit.c

Smatch reports the following:
  CHECK   drivers/staging/rtl8188eu/core/rtw_xmit.c
drivers/staging/rtl8188eu/core/rtw_xmit.c:1570 dequeue_one_xmitframe() info: ignoring unreachable code.

This code uses a while construction that has an unconditional break. This part
of the code was restructured to use an if statement and the dead code was
removed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>