libertas_tf: Fix possible memory leak in probe and disconnect

I got memory leak as follows when doing fault injection test:

unreferenced object 0xffff88810a2ddc00 (size 512):
  comm "kworker/6:1", pid 176, jiffies 4295009893 (age 757.220s)
  hex dump (first 32 bytes):
    00 50 05 18 81 88 ff ff 00 00 00 00 00 00 00 00  .P..............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff8167939c>] slab_post_alloc_hook+0x9c/0x490
    [<ffffffff8167f627>] kmem_cache_alloc_trace+0x1f7/0x470
    [<ffffffffa02a1530>] if_usb_probe+0x60/0x37c [libertas_tf_usb]
    [<ffffffffa022668a>] usb_probe_interface+0x1aa/0x3c0 [usbcore]
    [<ffffffff82b59630>] really_probe+0x190/0x480
    [<ffffffff82b59a19>] __driver_probe_device+0xf9/0x180
    [<ffffffff82b59af3>] driver_probe_device+0x53/0x130
    [<ffffffff82b5a075>] __device_attach_driver+0x105/0x130
    [<ffffffff82b55949>] bus_for_each_drv+0x129/0x190
    [<ffffffff82b593c9>] __device_attach+0x1c9/0x270
    [<ffffffff82b5a250>] device_initial_probe+0x20/0x30
    [<ffffffff82b579c2>] bus_probe_device+0x142/0x160
    [<ffffffff82b52e49>] device_add+0x829/0x1300
    [<ffffffffa02229b1>] usb_set_configuration+0xb01/0xcc0 [usbcore]
    [<ffffffffa0235c4e>] usb_generic_driver_probe+0x6e/0x90 [usbcore]
    [<ffffffffa022641f>] usb_probe_device+0x6f/0x130 [usbcore]

cardp is missing being freed in the error handling path of the probe
and the path of the disconnect, which will cause memory leak.

This patch adds the missing kfree().

Fixes: c305a19a ("libertas_tf: usb specific functions")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211020120345.2016045-2-wanghai38@huawei.com