HID: intel-ish-hid: ishtp: remove variable rb_count

The variable rb_count is being incremented but it
is never referenced, it is redundant and can be removed.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>