net: phy: dp83848: Reorganize code for readability and safety

Reorganize code by moving the desired interrupt mask definition
out of function. Also rearrange the enable/disable interrupt function
to prevent accidental over-writing of values in registers.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>