spi: pxa2xx: Rewrite switch code block in interrupt_transfer

Rewrite switch code block to directly do the expected number
of shifts in each case and have break statements.

Addresses-Coverity-ID: 1056539 ("Missing break in switch")
Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>