i2c: ismt: Fix undefined behavior due to shift overflowing the constant

Fix:

  drivers/i2c/busses/i2c-ismt.c: In function ‘ismt_hw_init’:
  drivers/i2c/busses/i2c-ismt.c:770:2: error: case label does not reduce to an integer constant
    case ISMT_SPGT_SPD_400K:
    ^~~~
  drivers/i2c/busses/i2c-ismt.c:773:2: error: case label does not reduce to an integer constant
    case ISMT_SPGT_SPD_1M:
    ^~~~

See https://lore.kernel.org/r/YkwQ6%2BtIH8GQpuct@zn.tnic

 for the gory
details as to why it triggers with older gccs only.

Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Seth Heasley <seth.heasley@intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>