i2c: mux: pca954x: Convert license to SPDX identifier

Use a shorter SPDX identifier instead of pasting the whole license.

While here, replace duplicating PCA954x with PCA984x.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>