soc: arm: stm32: fix sign of timeout variable

'timeout' needs to be a signed variable, so that the
check for less-than-zero is able to result to true (if
the timeout actually expires). Addresses the coverity
issue [Coverity CID :203493] reported in #18379.

Fixes #18379.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>