HID: usbhid: Simplify code in hid_submit_ctrl()

This patch makes a small simplification to the code in
hid_submit_ctrl().  The test for maxpacket being > 0 is unnecessary,
because endpoint 0 always has a maxpacket value which is >= 8.

Furthermore, endpoint 0's maxpacket value is always a power of 2, so
instead of open-coding the round-to-next-multiple computation we can
call the optimized round_up() routine.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>