staging: line6: Use kmemdup rather than duplicating its implementation

staging: line6: driver.c
 The semantic patch that makes this output is available
 in scripts/coccinelle/api/memdup.cocci.

Signed-off-by: Laurent Navet <laurent.navet@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>