diag: Fix incorrect length checks while reading data from peripherals

Diag driver reads mask table control packets from peripherals and
does a length check against an incorrect value. This can result in
dropping the last entry in those control packets.

Change-Id: I5092c6a7322c96bf7f6db51613b13d0b66977ffd
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>