drm/bridge: anx7625: check the return on anx7625_aux_trans

Clang static analysis reports this issue
anx7625.c:876:13: warning: The left operand of '&' is
  a garbage value
  if (!(bcap & 0xOA01)) {
        ~~~~ ^

bcap is only set by a successful call to
anx7625_aux_trans().  So check.

Fixes: cd1637c7

 ("drm/bridge: anx7625: add HDCP support")
Signed-off-by: Tom Rix <trix@redhat.com>
Fixes: adca62ec

 ("drm/bridge: anx7625: Support reading edid through aux channel")
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303201943.501746-1-trix@redhat.com
Reviewed-by: Robert Foss <robert.foss@linaro.org>