drivers: misc: pio_rpi_pico: fix linker error with -O0

The pio_rpi_pico_get_pio function was defined as 'inline' in the
header file. This caused 'undefined reference' linker errors when
building with CONFIG_NO_OPTIMIZATIONS=y (-O0), as the compiler
doesn't inline the function and no external symbol is provided.

This patch changes the definition to 'static inline' to ensure a
local copy is generated in each translation unit when not inlined,
consistent with other helper functions in Zephyr and the Linux kernel.

Fixes #101155

Signed-off-by: Vincent Liao <vincent932693@gmail.com>