Some HID devices are looking on the unused bits in the HID reports they
receive. This is violating the specification, but we want to make those
devices work. Well-behaving devices are unaffected, as they don't care
about the unused bits.

If bitsused % 8 is 0 all bits in data[] get used and we don't need to
clear anything. Otherwise (bitsused % 8) bits of the last byte get used.
By shifting 1 for (bitsused % 8) bits and subtracting 1 we create a mask
consisting of (bitsused % 8) ones and remaining zeroes. By ANDing we
clear the upper unused bits.
Signed-off-by: Simon Budig <simon@budig.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>