rtw89: pci: correct return value handling of rtw89_write16_mdio_mask()

Fix wrong checking statement. Fortunately, this wrong code doesn't affect
existing chip.

Fixes: 740c431c ("rtw89: pci: add register definition to rtw89_pci_info to generalize pci code")
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220413010804.8941-5-pkshih@realtek.com