staging/sep: Check pointers before dereferencing (fix smatch warning)

smatch complains about two dereferenced before check issues:

sep_main.c:2898 sep_free_dma_tables_and_dcb() warn: variable dereferenced before check
'dma_ctx' (see line 2885)
sep_main.c:2898 sep_free_dma_tables_and_dcb() warn: variable dereferenced before check
'*dma_ctx' (see line 2885)

-> Move the checks to the top, but keep the semantics.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>