staging: slicoss: handle allocation failure in slic_init_adapter

The memory allocation in slic_init_adapter() can fail. Return an error in
this case and unwind properly. Also make sure that the allocated memory is
properly freed in case of an error in the calling probe() function.
By doing this also replace the alloc() followed by memset to zero the
memory with a zalloc() version.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>