Buffer for SPI bus sysfs path too small

@kozga advised in issue #94 of a bug in
get_spi_bus_path_number()

> In version 1.0 declared path length is too low (50 bytes)
> But for kernels 4.1 path length is 54 bytes.
> In result, path is shortened to
> '/sys/devices/platform/ocp/48030000.spi/spi_master/'
> and function always return 1,
> even if you try to open /dev/spidev2.x.

These are the sysfs paths for the SPI bus 1 and 2
for Debian 8.6 with Linux 4.4.21-bone13 kernel:

/sys/devices/platform/ocp/48030000.spi/spi_master/spi1/
/sys/devices/platform/ocp/481a0000.spi/spi_master/spi2/

I have fixed by increasing the size of path to MAX_PATH.
MAX_PATH is defined as 256.  This should provide enough
extra margin for any future kernel changes in sysfs paths.

Author: pdp7

source/common.c

@@ -392,7 +392,7 @@ BBIO_err build_path(const char *partial_path, const char *prefix, char *full_pat
 
 int get_spi_bus_path_number(unsigned int spi)
 {
-  char path[50];
+  char path[MAX_PATH];
 
 #ifdef BBBVERSION41
   strncpy(ocp_dir, "/sys/devices/platform/ocp", sizeof(ocp_dir));

source/common.h

@@ -40,6 +40,7 @@ SOFTWARE.
 #define ARRAY_SIZE(a)  (sizeof(a) / sizeof(a[0]))
 
 #define FILENAME_BUFFER_SIZE 128
+#define MAX_PATH 256
 
 typedef enum {
 	BBIO_OK, // No error
@@ -91,4 +92,4 @@ BBIO_err get_pwm_by_key(const char *key, pwm_t **pwm);
 int setup_error;
 int module_setup;
 
-#endif
+#endif