wifi: mt76: Drop unnecessary error check for debugfs_create_dir()

debugfs_create_dir() returns ERR_PTR and never return NULL.

As Russell suggested, this patch removes the error checking for
debugfs_create_dir(). This is because the DebugFS kernel API is developed
in a way that the caller can safely ignore the errors that occur during
the creation of DebugFS nodes. The debugfs APIs have a IS_ERR() judge in
start_creating() which can handle it gracefully. So these checks are
unnecessary.

Fixes: 17f1de56 ("mt76: add common code shared between multiple chipsets")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Suggested-by: Russell King (Oracle) <linux@armlinux.org.uk>
Signed-off-by: Felix Fietkau <nbd@nbd.name>