net: 3com: 3c59x: Move boomerang/vortex conditional into function

If vp->full_bus_master_tx is set, vp->full_bus_master_rx is set as well
(see vortex_probe1()). Therefore the conditionals for the decision if
boomerang or vortex ISR is executed have the same result. Instead of
repeating the explicit conditional execution of the boomerang/vortex ISR,
move it into an own function.

No functional change.

Cc: Steffen Klassert <klassert@mathematik.tu-chemnitz.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>