em(4): Add support for Comet Lake Mobile Platform, update shared code

This change introduces Comet Lake Mobile Platform support in the e1000
driver along with shared code patches described below.

• Cast return value of e1000_ltr2ns() to higher type to avoid overflow
• Remove useless statement of assigning act_offset
• Add initialization of identification LED
• Fix flow control setup after connected standby: After connected standby the driver blocks resets during "AdapterStart" and skips flow control setup. This change adds condition in e1000_setup_link_ich8lan() to always setup flow control and to setup physical interface only when there is no need to block resets.

Signed-off-by: Piotr Pietruszewski <piotr.pietruszewski@intel.com>

Submitted by: Piotr Pietruszewski <piotr.pietruszewski@intel.com>
Reviewed by: erj@
Tested by: Jeffrey Pieper <jeffrey.e.pieper@intel.com>
MFC after: 1 week
Relnotes: yes
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D25035