rtlwifi: use work for lps

Leaving leisure power save mode can take some time, so it's better to
perform that action in process context with interrupts enabled. This
patch changes lps_leave tasklet to work.

Reported-by: Philipp Dreimann <philipp@dreimann.net>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Mike McCormack <mikem@ring3k.org>
Cc: Chaoming Li <chaoming_li@realsil.com.cn>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>