ibmvnic: Fix hardcoded NUM_RX_STATS/NUM_TX_STATS with dynamic sizeof

ibmvnic: Fix hardcoded NUM_RX_STATS/NUM_TX_STATS with dynamic sizeof

The previous hardcoded definitions of NUM_RX_STATS and
NUM_TX_STATS were not updated when new fields were added
to the ibmvnic_{rx,tx}_queue_stats structures. Specifically,
commit 2ee73c54a615 ("ibmvnic: Add stat for tx direct vs tx
batched") added a fourth TX stat, but NUM_TX_STATS remained 3,
leading to a mismatch.

This patch replaces the static defines with dynamic sizeof-based
calculations to ensure the stat arrays are correctly sized.
This fixes incorrect indexing and prevents incomplete stat
reporting in tools like ethtool.

Fixes: 2ee73c54a615 ("ibmvnic: Add stat for tx direct vs tx batched")
Signed-off-by: Mingming Cao <mmc@linux.ibm.com>
Reviewed-by: Dave Marquardt <davemarq@linux.ibm.com>
Reviewed-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250709153332.73892-1-mmc@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>