net-next: mediatek: remove superfluous register reads

The driver was originally written for MIPS based SoC. These required the
IRQ mask register to be read after writing it to ensure that the content
was actually applied. As this version only works on ARM based SoCs, we can
safely remove the 2 reads as they are no longer required.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>