test: dm_mdio: avoid out of bounds access

SANDBOX_PHY_REG_CNT is not an allowable index for the array
u16 reg[SANDBOX_PHY_REG_CNT].

Identified by cppcheck.

Fixes: b47edf80 ("test: dm_mdio: add a 2nd register to the emulated PHY")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>