drivers: clk: agilex: Use FIELD_GET during EMAC clock selection

drivers: clk: agilex: Use FIELD_GET during EMAC clock selection

FIELD_GET() macro is used during EMAC clock source selection
for better code readability and maintainability.

Signed-off-by: Naresh Kumar Ravulapalli <nareshkumar.ravulapalli@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>