clk: ti: Fix FAPLL parent enable bit handling

Commit 163152cb ("clk: ti: Add support for FAPLL on dm816x")
added basic support for the FAPLL on dm818x, but has a bug for the
parent PLL enable bit. The FAPLL_MAIN_PLLEN is defined as BIT(3)
but the code is doing a shift on it.

This means the parent PLL won't get disabled even if all it's child
synthesizers are disabled.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>