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>