staging: et131x: Replace kmem_cache use with plain kmalloc/kfree

The use of a kmem_cache was noted as being unusual in the TODO. Replace
the kmem_cache with kmalloc/kfree so that the code is less suprising.

Also tidy up the mess that was the et131x_init_recv() out of memory
error path.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>