mt76: fix debugfs_simple_attr.cocci warnings

drivers/net/wireless/mediatek/mt76/debugfs.c:36:0-23: WARNING: fops_regval should be defined with DEFINE_DEBUGFS_ATTRIBUTE

 Use DEFINE_DEBUGFS_ATTRIBUTE rather than DEFINE_SIMPLE_ATTRIBUTE
 for debugfs files.

Semantic patch information:
 Rationale: DEFINE_SIMPLE_ATTRIBUTE + debugfs_create_file()
 imposes some significant overhead as compared to
 DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe().

Generated by: scripts/coccinelle/api/debugfs/debugfs_simple_attr.cocci

Fixes: a5f6039c8f9c ("mt76: add driver code for MT76x2e")
CC: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>