bnxt_en: fix atomic counter for ptp packets

bnxt_en: fix atomic counter for ptp packets

atomic_dec_if_positive returns new value regardless if it is updated or
not. The commit in fixes changed the behavior of the condition to one
that differs from original code. Restore original condition to properly
maintain atomic counter.

Fixes: 165f87691a89 ("bnxt_en: add timestamping statistics support")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240604091939.785535-1-vadfed@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>