net: ks8851: Remove RXQCR cache

The cached RXQCR value is never updated, remove the cache and just use
the bits in the cache directly in the code.

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