We're doing some needless string copies when trying to assign the proper
`prop` string. We can make `prop` a const char* and simply assign to
string literals.

For the case where a format string is used, let's extract the parsing
logic out into sx9324_parse_phase_prop(). We no longer need to create
copies or allocate new memory.

sx9324_parse_phase_prop() will simply return the default def value if it
fails.

This also cleans up some deprecated strncpy() uses [1].

Furthermore, let's clean up this code further by removing some unused
defines:
|  #define SX9324_PIN_DEF "semtech,ph0-pin"
|  #define SX9324_RESOLUTION_DEF "semtech,ph01-resolution"
|  #define SX9324_PROXRAW_DEF "semtech,ph01-proxraw-strength"

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20240201-strncpy-drivers-iio-proximity-sx9324-c-v5-1-78dde23553bc@google.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>