radio: silabs: Fix the issue with current tuned station

Current tuned station not being updated after FM SEEK operation.
Save the current tuned station after successful FM SEEK operation.

Change-Id: Ifae8d3347b02a08d76c0f0c3178c946ad81bc72b
CRs-Fixed: 687224
Signed-off-by: Venkateshwarlu Domakonda <vdomak@codeaurora.org>