mfd: tps65218: Repair incorrect function argument name 's/tps65218/tps/'

The kerneldocs for both tps65218_reg_write() and tps65218_update_bits()
describe their first arguments as 'tps65218', when in reality these are
simply called 'tps'.

Fixes the following W=1 warnings:

 drivers/mfd/tps65218.c:58: warning: Function parameter or member 'tps' not described in 'tps65218_reg_write'
 drivers/mfd/tps65218.c:58: warning: Excess function parameter 'tps65218' description in 'tps65218_reg_write'
 drivers/mfd/tps65218.c:90: warning: Function parameter or member 'tps' not described in 'tps65218_update_bits'
 drivers/mfd/tps65218.c:90: warning: Excess function parameter 'tps65218' description in 'tps65218_update_bits'

Cc: Tony Lindgren <tony@atomide.com>
Cc: J Keerthy <j-keerthy@ti.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>