wl12xx: add support for fixed address in wl12xx_spi_read

In the wl1271 implementation, we need to read memory from the register
partition using fixed addresses.  This change adds the possibility to request
fixed address when calling wl12xx_spi_read() or wl12xx_spi_reg_read().

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>