wifi: rtlwifi: fix possible skb memory leak in _rtl_pci_init_one_rxdesc()

When `dma_mapping_error()` is true, if a new `skb` has been allocated,
then it must be de-allocated.

Compile tested only

Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250613074014.69856-2-fourier.thomas@gmail.com