net: dsa: mt7530: move p5_intf_modes() function to mt7530.c

In preparation of splitting mt7530.c into a driver for MDIO-connected
as well as MDIO-accessed built-in switches on one hand and MMIO-accessed
built-in switches move the p5_inft_modes() function from mt7530.h to
mt7530.c. The function is only needed there and will trigger a compiler
warning about a defined but unused function otherwise when including
mt7530.h in the to-be-introduced bus-specific drivers.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>