Avoid undefined behavior in RSNXE capability bit checker

Avoid undefined behavior in RSNXE capability bit checker

Integer promotion converts u8 rsnxe[i] to an int which is not
sufficiently large to be able to handle the maximum shift left of 24
bits here. Type cast rsnxe[i] to u32 explicitly to get rid of the sign
bit and avoid this undefined behavior from the shift operation.

Credit to OSS-Fuzz: https://issues.oss-fuzz.com/issues/376786400
Fixes: d675d3b15b40 ("Add helper functions for parsing RSNXE capabilities")
Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>