i2c: designware: make const array supported_speeds static to shink object code size

Don't populate const array supported_speeds on the stack, instead
make it static.  Makes the object code smaller by 150 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   8474	   1440	      0	   9914	   26ba	i2c-designware-platdrv.o

After:
   text	   data	    bss	    dec	    hex	filename
   8324	   1440	      0	   9764	   2624	i2c-designware-platdrv.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>