wl12xx: remove unused crc7 references

crc7 is used only in wl12xx_spi.

Remove redundant crc7.h includes, and update Kconfig to select CRC7
only if WL12XX_SPI is being selected.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>