e1000: always enable PCSD when RSS hashing

e1000: always enable PCSD when RSS hashing

To enable RSS hashing in the NIC, the PCSD bit must be set.

By default, this is never set when RXCSUM is disabled - which
causes problems higher up in the stack.

While here improve the RXCSUM flag assignments when enabling or
disabling IFCAP_RXCSUM.

See also: https://lists.freebsd.org/pipermail/freebsd-current/2020-May/076148.html

Reviewed by:	markj, Franco Fichtner <franco@opnsense.org>,
		Stephan de Wit <stephan.dewt@yahoo.co.uk>
Obtained from:	OPNsense
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D31501
Co-authored-by: Stephan de Wit <stephan.dewt@yahoo.co.uk>
Co-authored-by: Franco Fichtner <franco@opnsense.org>
(cherry picked from commit 69e8e8ea3d4be9da6b5bc904a444b51958128ff5)