wl1271: Implementation for SPI busy word checking

This patch adds implementation for checking for SPI busy words - i.e.
honoring a delay request from the WLAN chipset upon reading
registers/memory.

To optimized the average SPI ready by 32 bits, also configure the number
of busywords to one to disable the "fixed-busy-word" functionality.

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