macros for divisors and fix lf optimal freq display

include/pm3_cmd.h

@@ -578,6 +578,11 @@ typedef struct {
 // Quit program                         client:     reserved, order to quit the program
 #define PM3_EFATAL            -99
 
+// LF
+//#define LF_DIVISOR(f) ((12000 + (f)/2)/(f))
+//Note that 90 = 133.33 is closer to 134 than 89 = 134.83
+#define LF_DIVISOR_125 96
+#define LF_DIVISOR_134 89
 
 // Receiving from USART need more than 30ms as we used on USB
 // else we get errors about partial packet reception