app/testpmd: fix minor signed overflow in a constant

app/testpmd: fix minor signed overflow in a constant

The expression (192 << 24) has an undefined behavior since:
- the integer constant 192 has type int, and
- 192 x 2^24 is not representable as an int.

Suffixing 192 with U defines a behavior since:
- the integer constant 192U has type unsigned int, and
- the value of (192U << 24) is defined as
  (192 x 2^24) % (UINT_MAX + 1)

This minor bug was found using TrustInSoft Analyzer.

Signed-off-by: Julien Cretin <julien.cretin@trust-in-soft.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>