net: dc2114x: Split RX path

Split the RX data check from the rest of the RX function, so that
the check can be performed separately from the processing of the
packet and the release of the received packet once the processing
is finished.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>