e1000: Fix setting the promiscuous mode

The variable reg_rctl stores the value read from reg E1000_RCTL. It
may contain bits E1000_RCTL_VFE and E1000_RCTL_CFIEN which control
VLAN hardware filter feature. The promiscuous mode implies all tagged
or untagged packets should be accepted, so the VLAN hardware filter
feature should be disabled when enabling the promiscuous mode.
Calling em_if_vlan_filter_disable() did the task, but later writing
the value of reg_rctl back to the reg E1000_RCTL may restore the
feature.

Move the calling of em_if_vlan_filter_disable() after writing the reg
to fix that.

PR: 292759
Reviewed by: kbowling
Tested by: vova@zote.me
Fixes: 2796f7cab107 e1000: Fix up HW vlan ops
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54973