hwmon: (pmbus/isl68137) Fix unchecked return value and use sysfs_emit()

hwmon: (pmbus/isl68137) Fix unchecked return value and use sysfs_emit()

isl68137_avs_enable_show_page() uses the return value of
pmbus_read_byte_data() without checking for errors. If the I2C transaction
fails, a negative error code is passed through bitwise operations,
producing incorrect output.

Add an error check to propagate the return value if it is negative.
Additionally, modernize the callback by replacing sprintf()
with sysfs_emit().

Fixes: 038a9c3d1e424 ("hwmon: (pmbus/isl68137) Add driver for Intersil ISL68137 PWM Controller")
Cc: stable@vger.kernel.org
Signed-off-by: Sanman Pradhan <psanman@juniper.net>
Link: https://lore.kernel.org/r/20260318193952.47908-2-sanman.pradhan@hpe.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>