thunderbolt: Use tb_dp_read_cap() to read DP_COMMON_CAP as well

There is no point doing this separately as the register layout is the
same. For this reason rename tb_dp_read_dprx() to tb_dp_wait_dprx() and
call tb_dp_read_cap() instead.

While there add debug log if the DPRX capability read times out.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>