ACPI: pfr_update: Fix return value check in pfru_write()

In case of error, memremap() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.

Fixes: 0db89fa243e5 ("ACPI: Introduce Platform Firmware Runtime Update device driver")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Yang Yingliang <[e-mail]>
Acked-by: Chen Yu <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/acpi/pfr_update.c

···
 	/* map the communication buffer */
 	phy_addr = (phys_addr_t)((buf_info.addr_hi << 32) | buf_info.addr_lo);
 	buf_ptr = memremap(phy_addr, buf_info.buf_size, MEMREMAP_WB);
-	if (IS_ERR(buf_ptr))
-		return PTR_ERR(buf_ptr);
+	if (!buf_ptr)
+		return -ENOMEM;
 
 	if (!copy_from_iter_full(buf_ptr, len, &iter)) {
 		ret = -EINVAL;