drm/panic: clean Clippy warning

drm/panic: clean Clippy warning

Clippy warns:

    error: manual implementation of an assign operation
       --> drivers/gpu/drm/drm_panic_qr.rs:418:25
        |
    418 |                         self.carry = self.carry % pow;
        |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `self.carry %= pow`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assign_op_pattern

Thus clean it up.

Fixes: dbed4a797e00 ("drm/panic: Better binary encoding in QR code")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303093242.1011790-1-ojeda@kernel.org