wl1251: remove wl1251_ps_set_elp function

wl1251_ps_set_elp() only does acx_sleep_auth call and takes the chip
from/to ELP, however all callers of wl1251_ps_set_mode() have already
taken the chip out of ELP and puts it back to ELP when they finish.
This makes ELP calls (and register writes they result in) superfluous.

So remove wl1251_ps_set_elp function and call acx_sleep_auth directly.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>