zd1211rw: stop using deprecated get_seconds()

The get_seconds() function is deprecated because of the y2038 overflow.
In zd1211rw we don't even care about the absolute value, so this is
not a problem, but it's equally trivial to change to the non-deprecated
ktime_get_seconds().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>