qtnfmac: Missing platform_device_unregister() on error in qtnf_core_mac_alloc()

Add the missing platform_device_unregister() before return from
qtnf_core_mac_alloc() in the error handling case.

Fixes: 616f5701 ("qtnfmac: assign each wiphy to its own virtual platform device")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Reviewed-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200730064910.37589-1-wanghai38@huawei.com