[media] s5p: fix error code path when failing to allocate DMA memory

In the s5p_mfc_alloc_firmware() function there are some mistakes
where the code checks whether the DMA memory is properly allocated or
not.

First of all dma_alloc_coherent() returns NULL in case of error.
The code also checked two times fw_virt_addr, ignoring
the bank2_virt pointer.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>