net: arcnet: com20020-pci: avoid -Wformat-truncation warning

net: arcnet: com20020-pci: avoid -Wformat-truncation warning

When compiling the com20020-pci driver with W=1, I received the
following warning:

drivers/net/arcnet/com20020-pci.c:224:71: warning: ‘%d’ directive
output may be truncated writing between 1 and 11 bytes into a region of
size between 10 and 11 [-Wformat-truncation=]
 224 | snprintf(dev->name, sizeof(dev->name), "arc%d-%d", dev->dev_id, i);

In reality, this does not represent a problem, because i is bounded by
the .devcount field in struct com20020_pci_card_info, which is
statically defined for each card and very small. Quiet the invalid
warning by changing the type of i and the .devcount field to be
narrower.

Signed-off-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
Link: https://patch.msgid.link/20260521001631.45434-8-enelsonmoore@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>