mtd: rawnand: cortina_nand: Fix -ENOMEM detection

mtd: rawnand: cortina_nand: Fix -ENOMEM detection

In init_nand_dma there was code to detect failure to allocate memory but
it had two problems. Firstly the 2nd clause when info->tx_desc was NULL
attempted to free info->tx_desc when it should be freeing info->rx_desc.
Secondly there was no detection of both allocations failing, arguably
the more likely scenario. Refactor the code to simplify it and just fail
as soon as either allocation fails.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>