spi-topcliff-pch: fix -Wuninitialized warning

Fix for:
drivers/spi/spi-topcliff-pch.c: In function ‘pch_spi_handler_sub’:
drivers/spi/spi-topcliff-pch.c:325:17: warning: ‘bpw_len’ may be
  used uninitialized in this function [-Wuninitialized]
drivers/spi/spi-topcliff-pch.c:325:42: warning: ‘rx_index’ may be
  used uninitialized in this function [-Wuninitialized]
drivers/spi/spi-topcliff-pch.c:325:42: warning: ‘tx_index’ may be
  used uninitialized in this function [-Wuninitialized]

Move usage of tx_index, rx_index and bpw_len into the same
block as where they are set to prevent uninitialized usage.

v2: instead of init variables with 0 move the whole block

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>