phy: tegra: xusb: Uncomment register write

The reason why this was originally commented out is no longer clear. The
UPHY driver for SATA works fine with or without this change. The reset
value of the XDIGCLK_EN bit is 0, so unless programmed by the bootloader
this shouldn't make a difference anyway.

Define a macro for this bit and uncomment the code. This also fixes a
coverity issue brought to my attention by Rohith because not only is the
XDIGCLK_EN field modification commented out, but also the register write
which causes none of the earlier modifications of the register value to
be written to the register and the value being overwritten.

Reported-by: Rohith Seelaboyina <rseelaboyina@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>