lantiq: fix i2c driver compatibility issue on 6.12 kernel

lantiq: fix i2c driver compatibility issue on 6.12 kernel

* Drop obsolete flag I2C_CLASS_SPD. [1]
* Change .remove callback return type to NULL.
* Use strscpy instead of strlcpy.

This patch fixes the following build errors:

drivers/i2c/busses/i2c-lantiq.c:599:41: error: 'I2C_CLASS_SPD' undeclared (first use in this function); did you mean 'I2C_CLASS_HWMON'?
  599 |         adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;
      |                                         ^~~~~~~~~~~~~
      |                                         I2C_CLASS_HWMON
drivers/i2c/busses/i2c-lantiq.c:600:9: error: implicit declaration of function 'strlcpy'; did you mean 'strncpy'? [-Wimplicit-function-declaration]
  600 |         strlcpy(adap->name, DRV_NAME "-adapter", sizeof(adap->name));
      |         ^~~~~~~
      |         strncpy
drivers/i2c/busses/i2c-lantiq.c:729:19: error: initialization of 'void (*)(struct platform_device *)' from incompatible pointer type 'int (*)(struct platform_device *)' [-Wincompatible-pointer-types]
  729 |         .remove = ltq_i2c_remove,
      |

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-6.12.y&id=e61bcf42d290e73025bab38e0e55a5586c2d8ad5
Signed-off-by: Shiji Yang <[email protected]>