wifi: rtw88: fix incorrect error codes in rtw_debugfs_copy_from_user

If there is a failure during copy_from_user or user-provided data
buffer is invalid, rtw_debugfs_copy_from_user should return negative
error code instead of a positive value count.

Fix this bug by returning correct error code. Moreover, the check
of buffer against null is removed since it will be handled by
copy_from_user.

Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/tencent_D2EB102CC7435C0110154E62ECA6A7D67505@qq.com