iio: imu: inv_mpu6050_spi: Use vsprintf extension %pe for symbolic error name

Utilize %pe format specifier from vsprintf while printing error logs
with dev_err(). Discards the use of unnecessary explicit casting and
prints symbolic error name which might prove to be convenient during
debugging.

Signed-off-by: Nishant Malpani <nish.malpani25@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>