net: dsa: mv88e6xxx: fix SMI bit checking

The current mv88e6xxx_smi_direct_wait function is only used to check
the 16th bit of the (16-bit) SMI Command register. But the bit shift
operation is not enough if we eventually use this function to check
other bits, thus replace it with a mask.

Fixes: e7ba0fad

 ("net: dsa: mv88e6xxx: refine SMI support")
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>