[ARM] pxa: use 'pxa_last_gpio' instead of 'gpio_nr' in mfp-pxa2xx.c

The 'gpio_nr' can really be inferred by 'pxa_last_gpio', and since we
already have that variable, remove the unnecessary 'gpio_nr' now.

Also, fix the incorrect GPIO number passed in pxa27x_init_irq().

Note: pxa_last_gpio should be initialized earlier, and this is true
since it's been assigned in machine_desc->init_irq().

Signed-off-by: Eric Miao <eric.miao@marvell.com>