drivers: ethernet: eth_nxp_enet_qos: increase default buffer descriptors

Increased the default number of TX and RX buffer descriptors from 4 to
16. Since the current default buffer size (CONFIG_NET_BUF_DATA_SIZE) is
128, increasing the number of RX buffers is needed to be able to receive
at least one frame of a maximum size split between multiple buffers.

The default number of TX buffers was increased to match the number of RX
buffers and to be able to transmit large frames with many fragments.

Added build-time configuration validation to ensure that the combined
size of all RX buffers is sufficient to receive a maximum-sized Ethernet
frame.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>