mips: unify prom_putchar() declarations

prom_putchar() is used centrally in early printk infrastructure therefore
at least MIPS should agree on the function return type.

[paul.burton@mips.com:
  - Include linux/types.h in asm/setup.h to gain the bool typedef before
    we start include asm/setup.h elsewhere.
  - Include asm/setup.h in all files that use or define prom_putchar().
  - Also standardise on signed rather than unsigned char argument.]

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19842/
Cc: linux-mips@linux-mips.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>

arch/mips/ralink/early_printk.c

@@ -10,6 +10,7 @@
 #include <linux/serial_reg.h>
 
 #include <asm/addrspace.h>
+#include <asm/setup.h>
 
 #ifdef CONFIG_SOC_RT288X
 #define EARLY_UART_BASE		0x300c00
@@ -68,7 +69,7 @@ static void find_uart_base(void)
 	}
 }
 
-void prom_putchar(unsigned char ch)
+void prom_putchar(char ch)
 {
 	if (!init_complete) {
 		find_uart_base();
@@ -76,13 +77,13 @@ void prom_putchar(unsigned char ch)
 	}
 
 	if (IS_ENABLED(CONFIG_SOC_MT7621) || soc_is_mt7628()) {
-		uart_w32(ch, UART_TX);
+		uart_w32((unsigned char)ch, UART_TX);
 		while ((uart_r32(UART_REG_LSR) & UART_LSR_THRE) == 0)
 			;
 	} else {
 		while ((uart_r32(UART_REG_LSR_RT2880) & UART_LSR_THRE) == 0)
 			;
-		uart_w32(ch, UART_REG_TX);
+		uart_w32((unsigned char)ch, UART_REG_TX);
 		while ((uart_r32(UART_REG_LSR_RT2880) & UART_LSR_THRE) == 0)
 			;
 	}