wcn36xx: check for DMA mapping errors in wcn36xx_dxe_tx_frame()

Bail out if the mapping fails. Even though this hasn't occured during
tests, this unlikely case should still be handled.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Acked-by: Ramon Fried <rfried@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>