wl1271: Divided driver to two separate modules

Divided wl1271 driver to wl1271 "core" and wl1271_spi modules in preparation
of integration of the SDIO implementation.

Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>