eth: fbnic: Add ethtool support for IRQ coalescing

eth: fbnic: Add ethtool support for IRQ coalescing

Add ethtool support to configure the IRQ coalescing behavior. Support
separate timers for Rx and Tx for time based coalescing. For frame based
configuration, currently we only support the Rx side.

The hardware allows configuration of descriptor count instead of frame
count requiring conversion between the two. We assume 2 descriptors
per frame, one for the metadata and one for the data segment.

When rx-frames are not configured, we set the RX descriptor count to
half the ring size as a fail safe.

Default configuration:
ethtool -c eth0 | grep -E "rx-usecs:|tx-usecs:|rx-frames:"
rx-usecs:       30
rx-frames:      0
tx-usecs:       35

IRQ rate test:
With single iperf flow we monitor IRQ rate while changing the tx-usesc and
rx-usecs to high and low values.

ethtool -C eth0 rx-frames 8192 rx-usecs 150 tx-usecs 150
irq/sec   13k
irq/sec   14k
irq/sec   14k

ethtool -C eth0 rx-frames 8192 rx-usecs 10 tx-usecs 10
irq/sec  27k
irq/sec  28k
irq/sec  28k

Validating the use of extack:
ethtool -C eth0 rx-frames 16384
netlink error: fbnic: rx_frames is above device max
netlink error: Invalid argument

Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Link: https://patch.msgid.link/20250218023520.2038010-1-mohsin.bashr@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>