wcn36xx: release DMA memory in case of error

wcn36xx_dxe_init() doesn't check for the return value of
wcn36xx_dxe_init_descs(), release the resources in case an error ocurred.

Signed-off-by: Ramon Fried <rfried@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>