wl1271: Improve command polling

In testing I noticed that the wl1271 commands fall into two categories. In the
first category are "fast" commands, these mostly take only 0 or 1 polls to
complete, but occasionally upto 50 (giving a 0.5ms execution time.) In the
second category, the command completion takes well more than 0.5ms (from
1.5ms upwards.)

This patch fixes command polling such that it is optimal for the fast commands,
but also allows sleep for the longer ones.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>