tests: drivers: uart: uart_pm fix 'rxbuf' may be used uninitialized

Fix for uart_pm/src/main.c:106:23:
error: 'rxbuf' may be used uninitialized [-Werror=maybe-uninitialized]

string.h:62:10: note: by argument 2 of type 'const void *' to 'memcmp'
   62 | int      memcmp (const void *, const void *, size_t);
      |          ^~~~~~
tests/drivers/uart/uart_pm/src/main.c:69:17: note: 'rxbuf' declared here
   69 |         uint8_t rxbuf[32];
      |                 ^~~~~

Signed-off-by: Piotr Kosycarz <piotr.kosycarz@nordicsemi.no>