wl1251: retry power save entry

Port of the power save entry retry code from wl1251 driver version included
in the Maemo Fremantle kernel.
This tries to enable power save mode up to 3 times before failing.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>