stmmac: dwmac-ipq806x: fix static checker warning

The patch b1c17215: "stmmac: add ipq806x glue layer", leads to the
following static checker warning:

.../stmmac/dwmac-ipq806x.c:314 ipq806x_gmac_probe()
warn: double left shift '1 << (1 << gmac->id)'

The NSS_COMMON_CLK_SRC_CTRL_OFFSET macro is used once as an offset, and
once as a mask, which is a bug indeed. We'll fix it by defining the
offset as the real offset value and computing the mask from it when
required.

Tested on IPQ806x ref designs AP148 & DB149.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>