hwmon: (ads7871) Propagate SPI errors in voltage_show

hwmon: (ads7871) Propagate SPI errors in voltage_show

The voltage_show() function previously ignored negative error codes
returned by the underlying SPI read/write functions. Because negative
numbers have their most significant bits set in two's complement, a
failed SPI read returning -EIO (-5) would incorrectly evaluate to true
when masked with MUX_CNV_BM (0x80).

This would cause the driver to enter the polling loop even when the SPI bus
failed, eventually returning a misleading -ETIMEDOUT error to userspace
instead of the actual hardware error. Furthermore, the return values of
the initial SPI write and the final 16-bit SPI read were completely
ignored.

Add proper error checking after every SPI operation to ensure hardware
failures are immediately propagated back to userspace.

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
Link: https://lore.kernel.org/r/20260308124714.84715-1-tabreztalks@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>