wifi: ath5k: remove phydir check from ath5k_debug_init_device()

'phydir' returned from debugfs_create_dir() is checked against NULL.
As the debugfs API returns an error pointer,
the returned value can never be NULL.

Therefore, as the documentation suggests that the check is unnecessary
and other debugfs calls have no operation in error cases,
it is advisable to completely eliminate the check.

Signed-off-by: Minjie Du <duminjie@vivo.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230714081619.2032-1-duminjie@vivo.com