net: ks8851: Added support for half-duplex SPI

In original driver was implemented support for half-
and full-duplex modes, but it was not enabled. Instead
of it ks8851_rx_1msg method always returns "true" that
means "full-duplex" mode.

This patch replaces hard-coded functionality with
flexible solution that supports both SPI modes.

Signed-off-by: Sergey Shcherbakov <shchers@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>