gpiolib: Use sysfs_emit() in "show" functions

gpiolib: Use sysfs_emit() in "show" functions

The sysfs_emit() function was introduced to make it less ambiguous
which function is preferred when writing to the output buffer in
a "show" callback [1].

Convert the GPIO library sysfs interface from sprintf() to sysfs_emit()
accordingly, as the latter is aware of the PAGE_SIZE buffer and correctly
returns the number of bytes written into the buffer.

No functional change intended.

[1] Documentation/filesystems/sysfs.rst

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>