wl1251: move to it's own directory

wl1271 driver is under heavy development but on the other hand the older
wl1251 driver is currently considered more as a legacy driver. To make it
easier to develop wl1271 features move wl1251 to it's own directory,
drivers/net/wireless/wl1251.

There are no functional changes, only moving of files. One regression
is that Kconfig won't be updated automatically and user needs to enable
wl1251 manually with an older config file.

Signed-off-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>