reset: rzg2l-usbphy-ctrl: Check pwrrdy is valid before using it

reset: rzg2l-usbphy-ctrl: Check pwrrdy is valid before using it

The pwrrdy regmap_filed is allocated in rzg2l_usbphy_ctrl_pwrrdy_init()
only if the driver data is set to RZG2L_USBPHY_CTRL_PWRRDY. Check that
pwrrdy is valid before using it to avoid "Unable to handle kernel NULL
pointer dereference at virtual address" errors.

Fixes: c5b7cd9adefc ("reset: rzg2l-usbphy-ctrl: Add suspend/resume support")
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>