mx6: clock: Fix the logic for reading axi_alt_sel

mx6: clock: Fix the logic for reading axi_alt_sel

According to the IMX6DQRM Reference Manual, the description
of bit 7 (axi_alt_sel) of the CCM_CBCDR register is:

"AXI alternative clock select
0 pll2 396MHz PFD will be selected as alternative clock for AXI root clock
1 pll3 540MHz PFD will be selected as alternative clock for AXI root clock "

The current logic is inverted, so fix it to match the reference manual.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>