net: ks8851: Use 16bit RXQCR access

Per KS8851-16MLL, the RXQCR is a 16bit register. Use 16bit accessors
to it consistently and drop the ks_wrreg8() function altogether, as
it is not used anymore.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>