staging/easycap: Use memdup_user

 Use kmemdup_user rather than duplicating its implementation
 This is a little bit restricted to reduce false positives

 The semantic patch that makes this output is available
 in scripts/coccinelle/api/memdup_user.cocci.

 More information about semantic patching is available at
 http://coccinelle.lip6.fr/

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>