docs: kdoc_parser: use '@' for Excess enum value

docs: kdoc_parser: use '@' for Excess enum value

kdoc is looking for "@value" here, so use that kind of string in the
warning message. The "%value" can be confusing.

This changes:
Warning: drivers/net/wireless/mediatek/mt76/testmode.h:92 Excess enum value '%MT76_TM_ATTR_TX_PENDING' description in 'mt76_testmode_attr'

to this:
Warning: drivers/net/wireless/mediatek/mt76/testmode.h:92 Excess enum value '@MT76_TM_ATTR_TX_PENDING' description in 'mt76_testmode_attr'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251126061752.3497106-1-rdunlap@infradead.org>