cpu/lpc2387: Fixed bug and readability in IAP

- Previous cast was to a function pointer was not legal
- Using already present function pointer to store the IAP entry point to improve
  readability. (Which also fixes the cast issue.)

cpu/lpc2387/lpc23xx-iap.c

@@ -38,7 +38,7 @@ static unsigned int iap_command[5];
 static unsigned int iap_result[2];
 /* typedefinition for IAP entry function */
 typedef void (*IAP)(unsigned int[], unsigned int[]);
-IAP IAP_Entry;
+static const IAP IAP_Entry = (IAP)0x7ffffff1;
 
 /* some function prototypes */
 static uint32_t blank_check_sector(uint32_t tmp_sect1, uint32_t tmp_sect2);
@@ -157,7 +157,7 @@ static uint32_t iap(uint32_t code, uint32_t p1, uint32_t p2, uint32_t p3, uint32
     iap_command[3] = p3;        /* set 3rd param */
     iap_command[4] = p4;        /* set 4th param */
 
-    ((void (*)())0x7ffffff1)(iap_command, iap_result); /* IAP entry point */
+    IAP_Entry(iap_command, iap_result); /* IAP entry point */
     return *iap_result;
 }