usb: common: usb-conn-gpio: Simplify some error message

dev_err_probe() already prints the error code in a human readable way, so
there is no need to duplicate it as a numerical value at the end of the
message.

Reviewed-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/7505a9dfa1e097070c492d6f6f84afa2a490b040.1659763173.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>