soc: stm32f1: gpio: Fix unnecessary else statement

The bitfield determining the I/O direction already defines the pin
as either input or output, cannot be none or both at the same time

This issue was reported by Coverity

Coverity-CID: 151970

Change-Id: I18d5387139d6834004ba3269c5b54176bdc97ea7
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>