drivers/nble: Fix compare logic in on_nble_gattc_read_rsp

The Read Response only contains a Characteristic Value that is less
than or equal to (ATT_MTU – 1) octets in length so, condition
(length < bt_att_get_mtu(conn)) is always true, and the full
Characteristic Value will not be read.

Change-Id: Ib18fbedf277c880dc5cf0ce21fd7d550d12a64ef
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>