Fix mceusb_cmdsize() which returns incorrect datasize=0 for
sub-commands MCE_RSP_GETPORTSTATUS, MCE_RSP_GETWAKESOURCE,
MCE_RSP_EQDEVDETAILS, MCE_RSP_EQEMVER, and MCE_RSP_EQIRNUMPORTS.

While here, change mceusb_cmdsize() name to reflect that it returns
data size not cmd size.
Signed-off-by: William Steidtmann <billstei@hbci.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>