drivers: fpga: fix function declaration without a prototype

drivers: fpga: fix function declaration without a prototype

As reported by clang 20.1, fix multiple of the following:
 drivers/fpga/ivm_core.c:593:23: error: a function declaration without
 a prototype is deprecated in all versions of C
 [-Werror,-Wstrict-prototypes]
  593 | long int ispVMDataSize()
      |                       ^
      |                        void

Also fix the following warning from checkpatch.pl:
 WARNING: Prefer 'long' over 'long int' as the int is unnecessary

Signed-off-by: Pieter Van Trappen <pieter.van.trappen@cern.ch>
Link: https://lore.kernel.org/r/20250708152455.1214487-6-vtpieter@gmail.com
Signed-off-by: Michal Simek <michal.simek@amd.com>