wl12xx: add wl12xx_spi_reg_read() and wl12xx_spi_reg_write() functions

In some cases we need to read more than 32 bits from the register area.
These functions were added to support that, like the existing
wl12xx_spi_mem_read() and wl12xx_spi_mem_write() already do for large blocks
in the memory area.

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>