SPDY: Fix bitmask for priority bit

Found by yurui ming

Ping-Bug: 10566

Change-Id: I92e784eb91dc6c88af409e2936671b0ea473fba9
Reviewed-on: https://code.wireshark.org/review/4637
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit 017fc82d7404444ef43212be19f1307ad129ccf2)
Reviewed-on: https://code.wireshark.org/review/4648