greybus: use memdup_user()

Coccinelle reports that there are two opportunities to use memdup_user()
in "authentication.c".  This patch simplifies the code in cap_ioctl() by
taking advantage of that.  Make use of a local variable "size" to improve
readability.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>