[seeder] Fix the net magic not being found

Summary: Since D14689 the CDataStream content is always of type uint8_t. The seeder macros to check for the net magic casts to char, which causes a mismatch.

Test Plan: Checked live on a seeder that it fixes the issue.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14868