net: mscc: simplify retrieving the tag type from the frame header

The tag type in the frame extraction header is only a bit wide. There's
no need to use GENMASK when retrieving the information. This patch
simplify the code by dropping GENMASK and using BIT instead.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>