net: stmmac: replace the ext_snapshot_en field with a flag

Drop the boolean field of the plat_stmmacenet_data structure in favor of a
simple bitfield flag.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20230710090001.303225-10-brgl@bgdev.pl


Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>