smsc95xx: fix operation on 64-bit systems

smsc95xx_read_reg() should calculate sizeof(*data) not sizeof(data) since
data is a pointer, and the value pointed at is being transferred over USB,
not the value of the pointer. This fixes operation of the driver in 64-bit
builds, such as the Raspberry Pi 3.

Reported-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>