Staging: bcm: InterfaceTx.c: Outsourced code chunk

This patch outsources a chunk of code into an own function and refactors
the variable names inside the new function.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>
Acked-by: Kevin McKinney <klmckinney1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>