drivers: eth_mcux: Optimize check if received frame too large

For some reason, there was sequence like:

1. Get size of RX packet.
2. Allocate pkt buffer.
3. Check if the size of RX packet is too large, then deallocate pkt
buffer and error out.

Instead, reorder operations to check size before allocating buf.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>