Fix int size of mk_wheel_interval to match mousekey.c

[dcpedit]

Getting compile errors when enabling kinetic mode:

#define MK_KINETIC_SPEED

Error:

Compiling: quantum/mousekey.c                                                                      quantum/mousekey.c:83:10: error: conflicting types for 'mk_wheel_interval'; have 'uint16_t' {aka 'short unsigned int'}
   83 | uint16_t mk_wheel_interval = 1000U / MOUSEKEY_WHEEL_INITIAL_MOVEMENTS;
      |          ^~~~~~~~~~~~~~~~~
In file included from quantum/mousekey.c:25:
quantum/mousekey.h:186:16: note: previous declaration of 'mk_wheel_interval' with type 'uint8_t' {aka 'unsigned char'}
  186 | extern uint8_t mk_wheel_interval;
      |                ^~~~~~~~~~~~~~~~~
 [ERRORS]

Need to add logic around the int size of mk_wheel_interval to match what's in mousekey.c