tcpm/anx74xx: initialize reg variable before use
tcpm/anx74xx: initialize reg variable before use

When disabling auto_good_crc, the reg variable was being used
without initialization.  Mirror the code for enabling auto_good_crc
to set the variable.

TEST=Booted reef with updated code.
BUG=None
BRANCH=None

Change-Id: Ie552f2ff74df05750bd65b6344d8a80cc285f8b0
Signed-off-by: Martin Roth <79230511ccf79da3f7e9e82dbcc9c1ec2f734fa0@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/368221
Reviewed-by: David Hendricks <3b0deacc4b5357f519084f1ea34bd42bf5d34545@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec