radio: silabs: Unify silabs transport & radio drivers

Remove the usage of global variables.
Avoid sharing gpio information between two drivers.

CRs-Fixed: 684702
Change-Id: I65a9a7a326b395e5349614c78de8f4e91ac793e1
Signed-off-by: Venkateshwarlu Domakonda <vdomak@codeaurora.org>