wl1251: fix ELP_CTRL register reads

Reading the ELP_CTRL register with sdio_readb causes problems because
hardware seems to be performing a write using stuff bits in the request
(those bits contain write data in write request). This indicates that it
actually expects RAW (read after write) type of request, so perform that
when reading ELP_CTRL instead. Also cache last written value so we know
what to write when doing RAW request.

Because of the above it was not possible to wake the chip from ELP power
saving mode, PM had to be disabled to have the driver usable in SDIO
mode. After this patch PM is functional.

For backporting to 2.6.34 or earlier, this patch depends on
6c1f716e, which adds the
required SDIO funcion.

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