wl1251: fix TSF calculation

Cast MSB part of current TSF to u64 to prevent loss of most
significant bits. MSB should also be shifted by 32.

Patch based on old maemo patch by:
Yuri Kululin <ext-yuri.kululin@nokia.com>
Yuri Ershov <ext-yuri.ershov@nokia.com>

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>