ptp: lan743x: Use spin_lock instead of spin_lock_bh

lan743x_ptp_request_tx_timestamp uses spin_lock_bh, but it is
only called from lan743x_tx_xmit_frame where all IRQs are
already disabled.

This fixes the "IRQs not enabled as expected" warning.

Signed-off-by: Lucas Tanure <tanure@linux.com>
Link: https://lore.kernel.org/r/20240128101849.107298-1-tanure@linux.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>