net: ftmac100: prepare data path for receiving single segment packets > 1514

Eliminate one check in the data path and move it elsewhere, to where our
real limitation is. We'll want to start processing "too long" frames in
the driver (currently there is a hardware MAC setting which drops
theses).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Link: https://lore.kernel.org/r/20221028183220.155948-1-saproj@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>