platform/chrome: cros_ec_lpc: initialize the buf variable

platform/chrome: cros_ec_lpc: initialize the buf variable

Clang static analysis reports this problem
drivers/platform/chrome/cros_ec_lpc.c:379:13: warning: The left operand
  of '!=' is a garbage value [core.UndefinedBinaryOperatorResult]
    if (buf[0] != 'E' || buf[1] != 'C') {
         ~~~~~~ ^

The check depends on the side effect of the read.  When the read fails
or is short, a buf containing garbage could be mistaken as correct.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20230110193611.3573777-1-trix@redhat.com