HID: fix bug in zeroing the last field byte in output reports

d4ae650a904612ffb7edd3f28b69b022988d2466 introduced zeroing of the
last field byte in output reports in order to make sure the unused
bits are set to 0. This is done in a wrong way, resulting in a
wrong bits being zeroed out (not properly shifted by the field offset
in the report). Fix this.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>