drivers: nxp_enet: Fix CID 363724

This addresses the coverity issue ID 363724.
This issue seems to have been a false positive, but the root complaint
from coverity was that within the callstack of ENET_Up, some functions
use the buffer config pointer as an array, whereas in the zephyr driver
it is declared as a singleton. Address it by redeclaring this as an
array of size 1 instead of a singleton.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>