net: phy: bugfixes: mv88E61xx compiler warnings fixed

1. mv88E61xx driver compiler warnings fixed
2. idstr if-else statements changed to switch() construct
   and added default case too.
   This fixed idstr may be uninitialized warning

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>