nfc: trf7970a: power down on startup RX gain failure

In the Linux kernel, the following vulnerability has been resolved: nfc: trf7970a: power down on startup RX gain failure trf7970a_startup() powers up the device before applying the optional RX gain reduction. If the register read or write fails, it returns without undoing that power-up. Probe's unwind only drops the separate regulator references acquired by probe, leaving the additional VIN enable from startup unbalanced. The system resume caller also has no power-down on this error. Call trf7970a_power_down() before returning the RX gain error to deassert the enable GPIOs, release the startup VIN reference and restore the powered-off state. Runtime PM has not been enabled yet, so the full shutdown helper is not appropriate here. Preserve the original SPI error. This issue was identified during our ongoing static-analysis research while reviewing kernel code.