ptp_pch: Remove unused function 'pch_ch_control_read()'

Fixes the following W=1 kernel build warning(s):

 drivers/ptp/ptp_pch.c:182:5: warning: no previous prototype for ‘pch_ch_control_read’ [-Wmissing-prototypes]

Cc: Richard Cochran <richardcochran@gmail.com> (maintainer:PTP HARDWARE CLOCK SUPPORT)
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Flavio Suligoi <f.suligoi@asem.it>
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>