floss: hid: reply to get_report callback

Linux UHID driver requires an ack when a previous get_report request is
fulfilled; otherwise, it will wait 5 seconds to timeout. This patch
helps Floss send the response when the get_report callback gets
executed.

Bug: 243087440
Test: gd/cert/run
Test: HP910 reconnection time hugely reduced
Tag: #floss
Change-Id: I70e0268e6a03662ccc7f4d694069b4e63259d239