wifi: mwifiex: debugfs: 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: 5e6e3a92 ("wireless: mwifiex: initial commit for Marvell mwifiex driver")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Suggested-by: Russell King (Oracle) <linux@armlinux.org.uk>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20230903030216.1509013-3-ruanjinjie@huawei.com