Input: atmel_mxt_ts - fix invalid return from mxt_get_bootloader_version

The patch e57a66aa: "Input: atmel_mxt_ts - read and report
bootloader version" from May 18, 2014, leads to the following static
checker warning:

	drivers/input/touchscreen/atmel_mxt_ts.c:437 mxt_get_bootloader_version()
	warn: signedness bug returning '(-5)'

drivers/input/touchscreen/atmel_mxt_ts.c
   429  static u8 mxt_get_bootloader_version(struct mxt_data *data, u8 val)
   430  {
   431          struct device *dev = &data->client->dev;
   432          u8 buf[3];
   433
   434          if (val & MXT_BOOT_EXTENDED_ID) {
   435                  if (mxt_bootloader_read(data, &buf[0], 3) != 0) {
   436                          dev_err(dev, "%s: i2c failure\n", __func__);
   437                          return -EIO;
                                       ^^^^
This gets truncated into a number from 0-255 and anyway the caller
doesn't check for errors.

(reported by Dan Carpenter)

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>