swiotlb: clean up EXPORT_SYMBOL usage

Impact: cleanup

swiotlb uses EXPORT_SYMBOL in an inconsistent way. Some functions use
EXPORT_SYMBOL at the end of functions. Some use it at the end of
swiotlb.c.

This cleans up swiotlb to use EXPORT_SYMBOL in a consistent way (at
the end of functions).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>