USB: serial: cp210x: fix partnum regression

When adding GPIO support for the cp2105, the mentioned commit by Martyn
Welch introduced a query for the part number of the chip. Unfortunately
the driver aborts probing when this query fails, so currently the driver
can not be used with chips not supporting this query.
I have a data cable for Siemens mobile phones (ID 10ab:10c5) where this
is the case.
With this patch the driver can be bound even if the part number can not
be queried.

Fixes: cf5276ce ("USB: serial: cp210x: Adding GPIO support for CP2105")
Signed-off-by: Sebastian Frei <dr.nop@gmx.net>
[ johan: amend commit message; shorten error message and demote to
         warning; drop unnecessary move of usb_set_serial_data() ]
Cc: stable <stable@vger.kernel.org>	# 4.9
Signed-off-by: Johan Hovold <johan@kernel.org>