include: phy: fix NULL pointer check in phy_write()

phy_write() uses bus->write() instead of bus->read(). This means NULL
pointer pre-check needs to happen on bus->write instead of bus->read.

Signed-off-by: Thirupathaiah Annapureddy <thiruan@linux.microsoft.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>