char: xilinx_hwicap: Remove pointless local variables

Remove unused variables which generates these warnings:
[linux-4.10-rc5/drivers/char/xilinx_hwicap/buffer_icap.c:301]: (style)
Variable 'num_writes' is modified but its new value is never used.
[linux-4.10-rc5/drivers/char/xilinx_hwicap/buffer_icap.c:356]: (style)
Variable 'read_count' is modified but its new value is never used.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>