gve: Default to max_rx_buffer_size for DQO if device supported

gve: Default to max_rx_buffer_size for DQO if device supported

Change the driver's default behavior to prefer the largest available RX
buffer length supported by the device for DQO format, rather than always
using the hardcoded 2K default.

Previously, the driver would initialize with
`GVE_DEFAULT_RX_BUFFER_SIZE` (2K), even if the device advertised support
for a larger length (e.g., 4K).

Performance observations:
- With LRO disabled, we observed >10% improvement in RX single stream
throughput when MTU >=2048.
- With LRO enabled, we observed >10% improvement in RX single stream
throughput when MTU >=1460.
- No regressions were observed.

Signed-off-by: Ankit Garg <nktgrg@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Jordan Rhee <jordanrhee@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251106192746.243525-5-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>