HID: hidraw: Replace Confusing += Operator with =

Setting of the return value of hidraw_read() uses the += operator when
= is more appropriate.  There is no case where ret can be anything
other than zero when the assignment is made, making = equivalent to
+= and much more clear.

Signed-off-by: Alan Ott <alan@signal11.us>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>