net: phy: spi_ks8995: generalize creation of SPI commands

Prepare creating SPI reads and writes for other switch families.
The KS8995 family uses the straight forward
	<8bit CMD><8bit ADDR>
sequence.
To be able to support KSZ8795 family, which uses
	<3bit CMD><12bit ADDR><1 bit TR>
make the SPI command creation chip variant dependent.

Signed-off-by: Helmut Buchsbaum <helmut.buchsbaum@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>