wifi: wil6210: debugfs: use DEFINE_SHOW_ATTRIBUTE to simplify fw_capabilities/fw_version

Use DEFINE_SHOW_ATTRIBUTE helper macro to simplify the code.
No functional change.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220922142858.3250469-1-liushixin2@huawei.com