[ln788h] Make sure printfs get redirected to correct uart

[szupi-ipuzs]

Same problem as on ambz2 - the printfs must always write to whichever uart is set as logger, not to the default one.

[Bl00d-B0b]

Analysis

This PR has two independent changes:

1. Bounds check — if (port >= SER_PORT_NUM) return; in putchar_p(). Straightforward safety guard, no issues.

2. Variable rename — replaces the uart_print_port local variable with extern uint32_t lt_uart_port.

Problem with the rename

lt_uart_port is referenced as extern but is not defined anywhere in the LN882H family. On ambz2 it is defined in that family's own printf.c (uint8_t lt_uart_port = 2;), but no equivalent definition exists for LN882H. As-is this would produce a linker error.

Conflict with #373

PR #373 (now merged) restructured the LN882H logger setup around uart_print_port:

• printf.c owns the definition: uint8_t uart_print_port = LT_UART_DEFAULT_LOGGER;
• lt_init.c holds extern uint8_t uart_print_port; and sets it to UART1 in lt_init_log()

If this PR merges, uart_print_port disappears from printf.c while lt_init.c still references it — compile/link failure.

What a correct fix would look like

To align LN882H with the ambz2 pattern, lt_uart_port needs to be defined (not just declared) in LN882H, and lt_init.c updated to set lt_uart_port instead of uart_print_port. The bounds check can land independently and cleanly without any of this.