drm: samsung: Don't return error if RX fifo is not empty

In some cases, RX fifo gets not only correct data but also an unknown
data from panel side that makes RX fifo not emmpty. This commit is to
ignore the error if RX fifo is not empy after reading data from panel
successfully.

Bug: 207113942
Test: check read back data
Change-Id: I6294c93ed87335b44c3e879dc640e3632ddc761e
Signed-off-by: Ken Huang <kenbshuang@google.com>