gpio: stmpe: Check return value of stmpe_reg_read in stmpe_gpio_irq_sync_unlock

The stmpe_reg_read function can fail, but its return value is not checked
in stmpe_gpio_irq_sync_unlock. This can lead to silent failures and
incorrect behavior if the hardware access fails.

This patch adds checks for the return value of stmpe_reg_read. If the
function fails, an error message is logged and the function returns
early to avoid further issues.

Fixes: b888fb6f2a27 ("gpio: stmpe: i2c transfer are forbiden in atomic context")
Cc: [e-mail] # 4.16+
Signed-off-by: Wentao Liang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-stmpe.c

···
 		[REG_IE][CSB] = STMPE_IDX_IEGPIOR_CSB,
 		[REG_IE][MSB] = STMPE_IDX_IEGPIOR_MSB,
 	};
-	int i, j;
+	int ret, i, j;
 
 	/*
 	 * STMPE1600: to be able to get IRQ from pins,
···
 	 * GPSR or GPCR registers
 	 */
 	if (stmpe->partnum == STMPE1600) {
-		stmpe_reg_read(stmpe, stmpe->regs[STMPE_IDX_GPMR_LSB]);
-		stmpe_reg_read(stmpe, stmpe->regs[STMPE_IDX_GPMR_CSB]);
+		ret = stmpe_reg_read(stmpe, stmpe->regs[STMPE_IDX_GPMR_LSB]);
+		if (ret < 0) {
+			dev_err(stmpe->dev, "Failed to read GPMR_LSB: %d\n", ret);
+			goto err;
+		}
+		ret = stmpe_reg_read(stmpe, stmpe->regs[STMPE_IDX_GPMR_CSB]);
+		if (ret < 0) {
+			dev_err(stmpe->dev, "Failed to read GPMR_CSB: %d\n", ret);
+			goto err;
+		}
 	}
 
 	for (i = 0; i < CACHE_NR_REGS; i++) {
···
 		}
 	}
 
+err:
 	mutex_unlock(&stmpe_gpio->irq_lock);
 }