sim_imd: Properly calculate IMD sectsize.

The ImageDisk sectsize field was incorrectly set to the number of
bytes in the sector, when it should be an index as follows:

00 =  128 bytes/sector
01 =  256 bytes/sector
02 =  512 bytes/sector
03 = 1024 bytes/sector
04 = 2048 bytes/sector
05 = 4096 bytes/sector
06 = 8192 bytes/sector

Tested disk formatting on MS-DOS 1.25, Cromemco CP/M 2.2,
Cromemco CDOS, OASIS 5.6.

sim_imd.h

@@ -86,6 +86,8 @@ typedef struct {
 #define IMD_MODE_300K_MFM       4
 #define IMD_MODE_250K_MFM       5
 
+#define IMD_MAX_SECTSIZE        6
+
 #define IMD_MODE_FM(x)      (x <= IMD_MODE_250K_FM)
 #define IMD_MODE_MFM(x)     (x >= IMD_MODE_500K_MFM)