fix: Off-by-1 enum terminator value

[davidrea]

It looks like the NRF_MESH_DFU_END_ERROR_BANK_AND_DESTINATION_OVERLAP enum value was added to nrf_mesh_dfu_end_t but the NRF_MESH_DFU_END_ERROR__LAST value not updated.

This should be a pretty benign error as the enum is only statically bounds-checked against the type range, but correcting it protects future runtime boundary checks from spurious errors.

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[peterhinson]

@davidrea - good catch!