wl1271: Update TX packet life time handling with higher resolution time

This patch uses a higher precision timer to synchronize with the firmware
clock. Improved precision is needed as on some platforms a jiffy may be up to
tens of milliseconds, and the required precision is closer to TU's.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>