lan743x: clean up software_isr function

lan743x: clean up software_isr function

For no apparent reason, this function reads the INT_STS register, and
checks if the software interrupt bit is set. These things have already
been carried out by this function's only caller.

Clean up by removing the redundant code.

Tested-by: Sven Van Asbroeck <thesven73@gmail.com> # lan7430
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Link: https://lore.kernel.org/r/20201123191529.14908-1-TheSven73@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>