net: dsa: mv88e6xxx: prefix Port Jamming macros

For implicit namespacing and clarity, prefix the common Port Jamming
Control Register macros with MV88E6XXX_PORT_JAM_CTL and the ones which
differ between implementations with a chosen reference model
(e.g. MV88E6097_PORT_JAM_CTL.)

The 88E6390 family renamed the register to Flow Control and turned it
into an indirect table. Document that as well.

Document the register and prefer ordered hex masks values for all
Marvell 16-bit registers.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>