Staging: rtl8712: Use memdup_user() instead of copy_from_user()

Use memdup_user() to avoid its duplicated implementation and simplify
code. memdup_user() uses GFP_KERNEL instead of GFP_ATOMIC,
which is valid because copy_from_user() might sleep and it's useless
to make the allocation atomic. Found with coccinelle.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>