libertas: Fix possible memory leak in probe and disconnect

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

unreferenced object 0xffff88812c7d7400 (size 512):
  comm "kworker/6:1", pid 176, jiffies 4295003332 (age 822.830s)
  hex dump (first 32 bytes):
    00 68 1e 04 81 88 ff ff 01 00 00 00 00 00 00 00  .h..............
    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
    [<ffffffffa02c9873>] if_usb_probe+0x63/0x446 [usb8xxx]
    [<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: 876c9d3a ("[PATCH] Marvell Libertas 8388 802.11b/g USB driver")
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/20211020120345.2016045-3-wanghai38@huawei.com