What is the need of hh and h format specifiers?

hh is a length modifier that specifies the destination type of the argument. The default for conversion format specifier x is unsigned int*. With hh, it becomes unsigned char* or signed char*.

Refer to the table herein for more details.

&mac[0] is a pointer to an unsigned char.¹%hhx means the corresponding arguments points to an unsigned char. Use square pegs for square holes: the conversion specifiers in the format string must match the argument types.

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¹ Actually, &mac[0] is a pointer to a uint8_t, and %hhx is still wrong for uint8_t. It “works” in many implementations because uint8_t is the same as unsigned char in many implementations. But the proper format is "%" SCNx8, as in:

#include <inttypes.h>
…
scanf(mac_str, "%" SCNx8 "… rest of format string", &mac[0], … rest of arguments);