i2c: designware-pci: Use standard pattern for memory allocation

The pattern
	foo = kmalloc(sizeof(*foo), GFP_KERNEL);
has an advantage when foo type is changed. Since we are planning a such,
better to be prepared by using standard pattern for memory allocation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>