s390/setup: always inline gen_lpswe()

gen_lpswe() contains a BUILD_BUG_ON() statement which depends on a function
parameter. If the compiler decides to generate a not inlined function this
will lead to a build error, even if all call sites pass a valid parameter.

To avoid this always inline gen_lpswe().

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>