spi: bcm63xx-hsspi: Fix compiler warning

When build for arm64 target, comipler reports the following warning:
drivers/spi/bcm63xx_hsspi.c: In function ‘bcm63xx_hsspi_xfer_dummy_cs’:
include/linux/kernel.h:184:17: warning: comparison of distinct pointer
types lacks a cast
  184 |  (void) (&_min1 == &_min2);  \
      |                 ^~
drivers/spi/bcm63xx_hsspi.c:298:22: note: in expansion of macro ‘min’
  298 |   size_t curr_step = min(step_size, data_bytes);

This change fix this warning by casting the data_bytes to size_t.

Fixes: 0e144ec3 ("spi: bcm63xx-hsspi: Add prepend mode support")
Signed-off-by: William Zhang <william.zhang@broadcom.com>