net: phy: mv88e61xx.c : fixed build warning

following build warning was observed

mv88e61xx.c: In function ‘mv88e61xx_busychk’:
mv88e61xx.c:208: warning: dereferencing type-punned pointer will break strict-aliasing rules

This patch fixes the same
Patch tested for rd6281a board build

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